abstract stringlengths 3 192k | title stringlengths 4 857 |
|---|---|
the x-ray afterglow of short hard bursts (shbs) of gamma rays provides compelling evidence that shbs are produced by highly relativistic jets launched in the birth of rotationally powered millisecond pulsars in merger of neutron stars and/or in mass accretion on neutron stars in low mass x-ray binaries. gravitational wave detection of relatively nearby neutron star mergers by ligo-virgo, followed by far off-axis short grbs or orphan afterglows of beamed away shbs with an msp-like light curve will verify beyond doubt the neutron star merger origin of shbs. | the smoking guns of short hard gamma ray bursts |
a bright long gamma-ray burst grb 141207a was observed by the fermi gamma-ray space telescope and detected by both instruments onboard. the observations show that the spectrum in the prompt phase is not well described by the canonical empirical band function alone, and that an additional power-law component is needed. in the early phase of the prompt emission, a modified blackbody with a hard low-energy photon index (α = +0.2 to +0.4) is detected, which suggests a photospheric origin. in a finely time-resolved analysis, the spectra are also well fitted by the modified blackbody combined with a power-law function. we discuss the physical parameters of the photosphere such as the bulk lorentz factor of the relativistic flow and the radius. we also discuss the physical origin of the extra power-law component observed during the prompt phase in the context of different models such as leptonic and hadronic scenarios in the internal shock regime and synchrotron emission in the external forward shock. in the afterglow phase, the temporal and spectral behaviors of the temporally extended high-energy emission and the fading x-ray emission detected by the x-ray telescope on-board swift are consistent with synchrotron emission in a radiative external forward shock. | high-energy non-thermal and thermal emission from grb141207a detected by fermi |
we report on the x-ray dust-scattering features observed around the afterglow of the gamma-ray burst grb 160623a. with an xmm-newton observation carried out ∼2 d after the burst, we found evidence of at least six rings, with angular size expanding between ∼2 and 9 arcmin, as expected for x-ray scattering of the prompt gamma-ray burst (grb) emission by dust clouds in our galaxy. from the expansion rate of the rings, we measured the distances of the dust layers with extraordinary precision: 528.1 ± 1.2, 679.2 ± 1.9, 789.0 ± 2.8, 952 ± 5, 1539 ± 20 and 5079 ± 64 pc. a spectral analysis of the ring spectra, based on an appropriate dust-scattering model (bare-gr-b) and the estimated burst fluence, allowed us to derive the column density of the individual dust layers, which are in the range 7 × 1020-1.5 × 1022 cm-2. the farthest dust layer (i.e. the one responsible for the smallest ring) is also the one with the lowest column density and it is possibly very extended, indicating a diffuse dust region. the properties derived for the six dust layers (distance, thickness and optical depth) are generally in good agreement with independent information on the reddening along this line of sight and on the distribution of molecular and atomic gas. | behind the dust curtain: the spectacular case of grb 160623a |
we present multi-wavelength observations of the hyper-energetic gamma-ray burst (grb) 130907a, a swift-discovered burst with early radio observations starting at ≈4 hr after the γ-ray trigger. grb 130907a was also detected by the fermi/lat instrument and at late times showed a strong spectral evolution in x-rays. we focus on the early-time radio observations, especially at >10 ghz, to attempt to identify reverse shock signatures. while our radio follow-up of grb 130907a ranks among the earliest observations of a grb with the karl g. jansky very large array, we did not see an unambiguous signature of a reverse shock. while a model with both reverse and forward shock can correctly describe the observations, the data is not constraining enough to decide upon the presence of the reverse-shock component. we model the broadband data using a simple forward-shock synchrotron scenario with a transition from a wind environment to a constant density interstellar medium (ism) in order to account for the observed features. within the confines of this model, we also derive the underlying physical parameters of the fireball, which are within typical ranges except for the wind density parameter (a*), which is higher than those for bursts with wind-ism transition, but typical for the general population of bursts. we note the importance of early-time radio observations of the afterglow (and of well-sampled light curves) for unambiguously identifying the potential contribution of the reverse shock. | early-time vla observations and broadband afterglow analysis of the fermi/lat detected grb 130907a |
we present our extensive observational campaign on the swift-discovered grb 141121a, almost 10 years after its launch. our observations cover radio through x-rays and extend for more than 30 days after discovery. the prompt phase of grb 141121a lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is eγ,iso = 8.0 × 1052 erg. due to the long prompt duration, grb 141121a falls into the recently discovered class of ultra-long grbs (ul-grbs). peculiar features of this burst are (1) a flat early-time optical light curve and (2) a radio-to-x-ray rebrightening around three days after the burst. the latter is followed by a steep optical-to-x-ray decay and a much shallower radio fading. we analyze grb 141121a in the context of the standard forward-reverse shock (fs, rs) scenario and we disentangle the fs and rs contributions. finally, we comment on the puzzling early-time (t ≲ 3 days) behavior of grb 141121a, and suggest that its interpretation may require a two-component jet model. overall, our analysis confirms that the class of ul-grbs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful gamma-ray burst explosions, and future missions (like svom, xtide, or iss-lobster) will provide many more of such objects. | happy birthday swift: ultra-long grb 141121a and its broadband afterglow |
extended emission in short gamma-ray bursts (sgrbs) is a mystery. by conducting time-resolved spectral analyses of the nine brightest events observed by the swift-xrt, we classify the early x-ray emission of sgrbs into two types. one is the extended emission with exponentially rapid decay, which shows significant spectral softening for hundreds of seconds after the sgrb trigger and is also detected by the swift-bat. the other is a dim afterglow that only shows power-law decay over 104 s. the correlations between the temporal decay and spectral indices of the extended emissions are inconsistent with the α-β correlation expected for the high-latitude curvature emission from a uniform jet. the observed too-rapid decay suggests that the emission is from a photosphere or a patchy surface, and manifests the stopping via a central engine such as magnetic reconnection at the black hole. | x-raying extended emission and rapid decay of short gamma-ray bursts |
the nature of the shallow decay phase in the x-ray afterglow of the gamma-ray burst (grb) is not yet clarified. we analyse the data of early x-ray afterglows of 26 grbs triggered by burst alert telescope onboard neil gehrels swift observatory and subsequently detected by fermi large area telescope (lat) and/or imaging atmospheric cherenkov telescopes. it is found that nine events (including two out of three very-high-energy gamma-ray events) have no shallow decay phase and that their x-ray afterglow light curves are well described by single power-law model except for the jet break at later epoch. the rest are fitted by double power-law model and have a break in the early epoch (around ks), however, eight events (including a very-high-energy gamma-ray event) have the pre-break decay index larger than 0.7. we also analyse the data of well-sampled x-ray afterglows of grbs without lat detection and compare their decay properties with those of high-energy and very-high-energy gamma-ray events. it is found that for the gev/tev bursts, the fraction of events whose x-ray afterglows are described by single power law is significantly larger than those for non-gev/tev grbs. even if the gev/tev grbs have shallow decay phase, their decay slope tends to be steeper than non-gev/tev bursts, that is, they have less noticeable shallow decay phase in the early x-ray afterglow. a possible interpretation along with the energy injection model is briefly discussed. | less noticeable shallow decay phase in early x-ray afterglows of gev/tev-detected gamma-ray bursts |
the physical origins of gamma-ray burst (grb) prompt emission (photosphere or synchrotron) are still subject to debate, after more than five decades. here, we find that many of the observed characteristics of 15 long grbs, which have the highest prompt emission efficiency ϵγ(ϵγ≳ 80%), strongly support photosphere (thermal) emission origin, in the following ways: (1) the relation between e p and e iso is almost ${e}_{{\rm{p}}}\propto {({e}_{\mathrm{iso}})}^{1/4}$ and the dispersion is quite small; (2) the simple power-law shape of the x-ray afterglow light curves and the presence of significant reverse shock signals in the optical afterglow light curves; (3) the best fits using the cutoff power-law model for time-integrated spectra; and (4) the consistent efficiency from observations (with e iso/ek ) and the predictions from the photosphere emission model (with η/γ). we then further investigate the characteristics of the long grbs for two distinguished samples (ϵγ≳ 50% and ϵγ≲ 50%). it is found that the different distributions for e p and e iso, and the similar observed efficiency (from the x-ray afterglow) and theoretically predicted efficiency (from the prompt emission or the optical afterglow), follow the predictions of the photosphere emission model well. also, based on the same efficiency, we derive an excellent correlation of ${\rm{\gamma }}\propto {e}_{\mathrm{iso}}^{1/8}{e}_{{\rm{p}}}^{1/2}/{\left({t}_{90}\right)}^{1/4}$ to estimate γ. finally, we show that different distributions for e p and e iso, and the consistent efficiency, exist for short grbs. we also give a natural explanation of the extended emission (ϵγ≲ 50%) and the main pulse (ϵγ≳ 50%). | evidence of photosphere emission origin for gamma-ray burst prompt emission |
recently, the magic collaboration reported a ~5σ statistical significance of the very-high-energy (vhe) emission from a distant gamma-ray burst (grb), grb 201216c. such distant grb may be effectively absorbed by the extragalactic background light (ebl). the origin of the vhe emission from such distant objects is still unknown. here, we propose a numerical model for studying the afterglow emission of this distant grb. the model solves the continuity equation governing the temporal evolution of electron distribution, and the broadband observed data can be explained by the synchrotron plus synchrotron self-compton (ssc) radiation of the forward shock. the predicted observed 0.1 tev flux can reach ~10-9-10-10 erg cm-2 s-1 at t ~ 103-104 s, even with strong ebl absorption, such strong sub-teraelectronvolt (sub-tev) emissions still can be observed by the magic telescope. using this numerical model, the shock parameters in the modeling are similar to two other sub-tev grbs (i.e., grb 190114c and grb 180720b), implying that the sub-tev grbs have some commonalities: they have energetic burst energy, low circumburst medium density, and a low magnetic equipartition factor. we regard grb 201216c as a typical grb, and estimate the maximum redshift of grb that can be detected by the magic telescope, i.e., z ~ 1.6. we also find that the vhe photon energy of such distant grb can only reach ~0.1 tev. improving the low energy sensitivity of the vhe telescope is very important to detect the sub-tev emissions of these distant grbs. | time-dependent numerical model for studying the very-high-energy emissions of distant gamma-ray burst grb 201216c |
we present the spectroscopic and photometric late-time follow-up of the host galaxy of the long-duration swift γ-ray burst grb 140506a at z = 0.889. the optical and near-infrared afterglow of this grb had a peculiar spectral energy distribution (sed) with a strong flux-drop at 8000 å (4000 å rest-frame) suggesting an unusually steep extinction curve. by analysing the contribution and physical properties of the host galaxy, we here aim at providing additional information on the properties and origin of this steep, non-standard extinction. we find that the strong flux-drop in the grb afterglow spectrum at <8000 å and rise at <4000 å (observers frame) is well explained by the combination of a steep extinction curve along the grb line of sight and contamination by the host galaxy light at short wavelengths so that the scenario with an extreme 2175 å extinction bump can be excluded. we localise the grb to be at a projected distance of approximately 4 kpc from the centre of the host galaxy. based on emission-line diagnostics of the four detected nebular lines, hα, hβ, [o ii] and [o iii], we find the host to be a modestly star forming (sfr = 1.34 ± 0.04 m⊙ yr-1) and relatively metal poor (z=0.35+0.15-0.11 z⊙) galaxy with a large dust content, characterised by a measured visual attenuation of av = 1.74 ± 0.41 mag. we compare the host to other grb hosts at similar redshifts and find that it is unexceptional in all its physical properties. we model the extinction curve of the host-corrected afterglow and show that the standard dust properties causing the reddening seen in the local group are inadequate in describing the steep drop. we thus conclude that the steep extinction curve seen in the afterglow towards the grb is of exotic origin and issightline-dependent only, further confirming that this type of reddening is present only at very local scales and that it is solely a consequence of the circumburst environment. based on observations carried out under programme ids 095.d-0043(a, c) and 095.a-0045(a) with the x-shooter spectrograph and the focal reducer and low dispersion spectrograph 2 (fors2) installed at the cassegrain very large telescope (vlt), unit 2 - kueyen and unit 1 - antu, respectively, operated by the european southern observatory (eso) on cerro paranal, chile.the reduced spectra (fits files) are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/601/a83 | steep extinction towards grb 140506a reconciled from host galaxy observations: evidence that steep reddening laws are local |
it is largely recognized that gamma-ray burst (grb) jets involve ultrarelativistic motion. however, the value of the lorentz factor γ0 is still not clear and only lower limits are known for most bursts. we suggest here a new method to obtain upper limits on γ0. the early high-energy synchrotron afterglow flux depends strongly on γ0. upper limits on gev emission therefore provide upper limits on γ0. applying this method to 190 fermi grbs which have not been detected by the fermi-lat, we place upper limits on the high-energy afterglow flux, and in turn on γ0. for bursts at a typical redshift z = 2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. these upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical lorentz factors of grb jets are of the order of a few hundred. | constraints on the bulk lorentz factor of gamma-ray burst jets from fermi /lat upper limits |
correlations between intrinsic properties of gamma-ray burst (grb) light curves provide clues to the nature of the central engine, the jet, and a possible means to standardize grbs for cosmological use. here, we report on the discovery of a correlation between the intrinsic early-time luminosity, lg,10 s, measured at rest frame 10 s, and the average decay rate measured from rest frame 10 s onward, $\alpha _{\mathrm{g,avg\gt 10\, s}}$, in a sample of 13 fermi large area telescope long grb light curves. we note that our selection criteria, in particular the requirement for a redshift to construct luminosity light curves, naturally limits our sample to energetic grbs. a spearman's rank correlation gives a coefficient of -0.74, corresponding to a confidence level of 99.6 per cent, indicating that brighter afterglows decay faster than less luminous ones. assuming a linear relation with log(lg,10s), we find $\alpha _{\mathrm{g,avg\gt 10\, s}}$$= -0.31_{-0.09}^{+0.12}\log ($lg,10s$) + 14.43_{-5.97}^{+4.55}$. the slope of -0.31 is consistent at 1σ with previously identified correlations in the optical/ultraviolet and x-ray light curves. we speculate that differences in the rate at which energy is released by the central engine or differences in observer viewing angle may be responsible for the correlation. | evidence for a luminosity-decay correlation in grb gev light curves |
aims: following the detection of the fast radio burst frb150418 by the superb project at the parkes radio telescope, we aim to search for very-high energy gamma-ray afterglow emission.methods: follow-up observations in the very-high energy gamma-ray domain were obtained with the h.e.s.s. imaging atmospheric cherenkov telescope system within 14.5 h of the radio burst.results: the obtained 1.4 h of gamma-ray observations are presented and discussed. at the 99% c.l. we obtained an integral upper limit on the gamma-ray flux of φγ(e > 350 gev) < 1.33 × 10-8 m-2 s-1. differential flux upper limits as function of the photon energy were derived and used to constrain the intrinsic high-energy afterglow emission of frb 150418.conclusions: no hints for high-energy afterglow emission of frb 150418 were found. taking absorption on the extragalactic background light into account and assuming a distance of z = 0.492 based on radio and optical counterpart studies and consistent with the frb dispersion, we constrain the gamma-ray luminosity at 1 tev to l < 5.1 × 1047 erg/s at 99% c.l. | first limits on the very-high energy gamma-ray afterglow emission of a fast radio burst. h.e.s.s. observations of frb 150418 |
the typical detection rate of ~1 gamma-ray burst (grb) per day by the fermi gamma-ray burst monitor (gbm) provides a valuable opportunity to further our understanding of grb physics. however, the large uncertainty of the fermi localization typically prevents rapid identification of multiwavelength counterparts. we report the follow-up of 93 fermi grbs with the gravitational-wave optical transient observer (goto) prototype on la palma. we selected 53 events (based on favourable observing conditions) for detailed analysis, and to demonstrate our strategy of searching for optical counterparts. we apply a filtering process consisting of both automated and manual steps to 60 085 candidates initially, rejecting all but 29, arising from 15 events. with ≈3 grb afterglows expected to be detectable with goto from our sample, most of the candidates are unlikely to be related to the grbs. since we did not have multiple observations for those candidates, we cannot confidently confirm the association between the transients and the grbs. our results show that goto can effectively search for grb optical counterparts thanks to its large field of view of ≈40 deg2 and its depth of ≈20 mag. we also detail several methods to improve our overall performance for future follow-up programmes of fermi grbs. | searching for fermi grb optical counterparts with the prototype gravitational-wave optical transient observer (goto) |
grb 220426a is a bright gamma-ray burst (grb) dominated by the photospheric emission. we perform several tests to speculate the origin of this photospheric emission. the dimensionless entropy η is large, which is not usual if we assume that it is a pure hot fireball launched by neutrino-antineutrino annihilation mechanism only. moreover, the outflow has larger η with lower luminosity l in the first few seconds, so that the trend of time-resolved η - l can not be described as a monotonically positive correlation between η and l. a hybrid outflow with almost completely thermalized poynting flux could account for the quasi-thermal spectrum as well as large η. more importantly, the existence of magnetic field could affect the proton density and neutron-proton coupling effect, so that it could account for the observed trend of time-resolved η - l. the other origins for the photospheric emission, such as non-dissipative hybrid outflow or magnetic reconnection, are not supported because their radiation efficiencies are low, which is not consistent with non-detection of the afterglow for grb 220426a. therefore, we think the hybrid outflow may be the most likely origin. | the origin of the photospheric emission of grb 220426a |
a relativistic electron-positron (e+e-) pair wind from a rapidly rotating, strongly magnetized neutron star (ns) would interact with a gamma-ray burst (grb) external shock and reshape afterglow emission signatures. assuming that the merger remnant of gw170817 is a long-lived ns, we show that a relativistic e+e- pair wind model with a simple top-hat jet viewed off-axis can reproduce multiwavelength afterglow lightcurves and superluminal motion of grb 170817a. the markov chain monte carlo (mcmc) method is adopted to obtain the best-fitting parameters, which give the jet half-opening angle θj ≈ 0.12 rad, and the viewing angle θv ≈ 0.23 rad. the best-fitting value of θv is close to the lower limit of the prior that is chosen based on the gravitational-wave and electromagnetic observations. in addition, we also derive the initial lorentz factor γ0 ≈ 49 and the isotropic kinetic energy ek,iso ≈ 1× 1052 erg. consistency between the corrected on-axis values for grb 170817a and typical values observed for short grbs indicates that our model can also reproduce the prompt emission of grb 170817a. an ns with a magnetic field strength bp ≈ 1.6 × 1013 g is obtained in our fitting, indicating that a relatively low thermalization efficiency η ≲ 10-3 is needed to satisfy observational constraints on the kilonova. furthermore, our model is able to reproduce a late-time shallow decay in the x-ray lightcurve, and predicts that the x-ray and radio flux will continue to decline in the coming years. | grb 170817a afterglow from a relativistic electron-positron pair wind observed off-axis |
the event gw170817/grb 170817a, discovered via the successful joint observation of its gravitational wave radiation and its multiwavelength electromagnetic counterparts, was the first definite `smoking-gun' from the merger of two neutron stars (nss). however, the remnant of the merger remains unknown. piro et al. recently claimed that a low-significance x-ray variability in grb 170817a. by systematically comparing the properties of variability in the afterglow of grb 170817a and x-ray flares in grb afterglows, we find that this x-ray variability seems to share similar statistical correlations with x-ray flares in grb afterglows. we further investigate several possible merger product scenarios to see whether they can produce the observed x-ray variability in grb 170817a. the first scenario invokes a stable magnetar as the central engine producing the later x-ray variability via differential rotation or fallback accretion onto the ns. the second scenario invokes a black hole as the central engine with a fallback accretion process. the final scenario is a central engine with a long-lived supra-massive ns. we find that the first two scenarios have difficulty producing the later x-ray variability, which requires either an impractical ns magnetic field or an extraordinarily large stellar envelope and an extremely long accretion time-scale. however, the third scenario seems to be consistent with observations, and the later x-ray variability can be produced by the magnetosphere which is expelled following the collapse of the ns with a bp ∈ (3.6, 13.5) × 1013 g. | diagnosing the remnants of binary neutron star merger from gw170817/grb170817a event |
the late-time high-resolution x-ray and radio observations of grb980425/sn1998bw, the closest known gamma ray burst (grb) associated with a supernova (sn) explosion, may have actually resolved the hyperluminal source that produced the grb and its afterglow. its hyperluminal speed ~350c is consistent with that expected in the cannonball (cb) model of grbs. the observed superluminal expansion of the late-time radio image of grb030329/sn2003dh, the grb with the brightest and longest followed up radio afterglow to date, is also consistent with that expected in the cb model of grbs and extrapolates to an apparent early-time hyperluminal expansion. | hyperluminal signatures in the afterglows of gamma-ray bursts 980425 and 030329 |
flares in the x-ray afterglow of gamma-ray bursts (grbs) share more characteristics with the prompt emission than the afterglow, such as pulse profile and contained fluence. as a result, they are believed to originate from late-time activity of the central engine and can be used to constrain the overall energy budget. in this paper, we collect a sample of 19 long grbs observed by swift-xrt that contain giant flares in their x-ray afterglows. we fit this sample with a version of the magnetar propeller model, modified to include fallback accretion. this model has already successfully reproduced extended emission in short grbs. our best-fittings provide a reasonable morphological match to the light curves. however, 16 out of 19 of the fits require efficiencies for the propeller mechanism that approach 100 per cent. the high-efficiency parameters are a direct result of the high energy contained in the flares and the extreme duration of the dipole component, which forces either slow spin periods or low-magnetic fields. we find that even with the inclusion of significant fallback accretion, in all but a few cases it is energetically challenging to produce prompt emission, afterglow, and giant flares within the constraints of the rotational energy budget of a magnetar. | fallback accretion on to a newborn magnetar: long grbs with giant x-ray flares |
twenty years have already been elapsed from the bepposax discovery of the first afterglow of a gamma-ray burst (grb) in february 28, 1997. thanks to this discovery, it was possible to unveil the 30 year mystery about grb origin: grbs are huge explosions in galaxies at cosmological distances. starting from the first grb detection with vela satellites, i will review the main results obtained before bepposax, the story of the bepposax discovery of the grb afterglow with the consequent cosmological distance determination, the main bepposax results, and those obtained after the bepposax era until nowadays with the still open problems and prospects. | the key role of bepposax in the grb history |
a structured jet is invoked to explain the afterglows and also the prompt emission of grb 170817a. in this article, we stress that, for a precessing jet, the jet structure in the prompt emission phase and that in the afterglow phase may be different. the jet structure in the afterglow phase can be non-uniform even if a narrow uniform jet is presented in the prompt emission phase. we estimate the jet structure in the afterglow phase in the situation in which a narrow uniform precessing jet is launched from the central engine of the gamma-ray burst. with different precession angles, it is found that the structured jet can be roughly described as follows: a narrow uniform core with power-law wings and sharp cut-off edges, a gaussian profile, a ring shape or another complex profile in energy per solid angle. correspondingly, the afterglows for our structured jets are also estimated. we find that estimates of the intrinsic kinetic energy, the electron index and the jet opening angle based on the afterglows formed in a precessing system may be incorrect. our obtained structured jet is likely to be revealed by future observations for a fraction of gravitational-wave-detected merging compact binary systems (e.g. black hole-neutron star mergers). | jet structure in the afterglow phase for gamma-ray bursts with a precessing jet |
we have developed a model of early x-ray afterglows of gamma-ray bursts originating from the reverse shock (rs) propagating through ultrarelativistic, highly magnetized pulsar-like winds produced by long-lasting central engines. we first performed fluid and magnetohydrodynamic numerical simulations of relativistic double explosions. we demonstrate that even for constant properties of the wind a variety of temporal behaviors can be produced, depending on the energy of the initial explosion and the wind power, the delay time for the switch-on of the wind, and the magnetization of the wind. x-ray emission of the highly magnetized rs occurs in the fast-cooling regime—this ensures high radiative efficiency and allows fast intensity variations. we demonstrate that (i) rs emission naturally produces light curves, showing power-law temporal evolution with various temporal indices; (ii) mild wind power, of the order of ∼1046 erg s-1 (equivalent isotropic), can reproduce the afterglows' plateau phase; (iii) termination of the wind can produce sudden steep decays; and (iv) short-duration afterglow flares are due to mild variations in the wind luminosity, with small total injected energy. | dynamics and emission of wind-powered afterglows of gamma-ray bursts: flares, plateaus, and steep decays |
very long and precise follow-up measurements of the x-ray afterglow of very intense gamma-ray bursts (grbs) allow a critical test of grb theories. here we show that the single power-law decay with time of the x-ray afterglow of grb 130427a, the record-long and most accurately measured x-ray afterglow of an intense grb by the swift, chandra, and xmm-newton space observatories, and of all other known intense grbs, is that predicted by the cannonball model of grbs from their measured spectral index, while it disagrees with that predicted by the widely accepted fireball models of grbs. | critical test of gamma-ray burst theories |
we present photometric and polarimetric measurements of gamma-ray burst (grb) optical afterglows observed by the ringo3 imaging polarimeter over its ~7 yr lifetime mounted on the liverpool telescope. during this time, ringo3 responded to 67 grb alerts. of these, 28 had optical afterglows and a further ten were sufficiently bright for photometric and polarimetric analysis (r ⪅ 17). we present high quality multicolour light curves of ten sources: grb 130606a, grb 130610a, grb 130612a, grb 140430a, grb 141220a, grb 151215a, grb 180325a, grb 180618a, grb 190114c, and grb 191016a and polarimetry for seven of these (excluding grb 130606a, grb 130610a, and grb 130612a, which were observed before the polarimetry mode was fully commissioned). eight of these ten grbs are classical long grbs, one sits at the short-long duration interface with a t90 ~ 4 s and one is a classical short, hard burst with extended emission. we detect polarization for grb 190114c and grb 191016a. while detailed analyses of several of these grbs have been published previously, here we present a uniform re-reduction and analysis of the whole sample and investigation of the population in a broad context relative to the current literature. we use survival analysis to fully include the polarization upper limits in comparison with other grb properties, such as temporal decay rate, isotropic energy, and redshift. we find no clear correlation between polarization properties and wider sample properties and conclude that larger samples of early time polarimetry of grb afterglows are required to fully understand grb magnetic fields. | polarimetry and photometry of gamma-ray bursts afterglows with ringo3 |
one of the cornerstones of modern physics is einstein's special relativity, with its constant speed of light and zero photon mass assumptions. constraint on the rest mass mγ of photons is a fundamental way to test einstein's theory, as well as other essential electromagnetic and particle theories. since non-zero photon mass can give rise to frequency- (or energy-) dependent dispersions, measuring the time delay of photons with different frequencies emitted from explosive astrophysical events is an important and model-independent method to put such a constraint. the cosmological gamma-ray bursts (grbs), with short time scales, high redshifts as well as broadband prompt and afterglow emissions, provide an ideal testbed for mγ constraints. in this paper we calculate the upper limits of the photon mass with grb early time radio afterglow observations as well as multi-band radio peaks, thus improve the results of schaefer (1999) by nearly half an order of magnitude. | constraining the mass of the photon with gamma-ray bursts |
using the generic hydrodynamic model of gamma-ray burst (grb) afterglows, we calculate the radio afterglow light curves of low luminosity, high luminosity, failed and standard grbs in different observational bands of fast's energy window. the grbs are assumed to be located at different distances from us. our results rank the detectability of grbs in descending order as high luminosity, standard, failed and low luminosity grbs. we predict that almost all types of radio afterglows except those of low luminosity grbs could be observed by a large radio telescope as long as the domains of time and frequency are appropriate. it is important to note that fast can detect relatively weak radio afterglows at a higher frequency of 2.5 ghz for very high redshift up to z = 15 or even more. radio afterglows of low luminosity grbs can only be detected after the completion of the second phase of fast. fast is expected to significantly expand the sample of grb radio afterglows in the near future. | detecting radio afterglows of gamma-ray bursts with fast |
the microchannel x-ray telescope (mxt) is part of the scientific payload of the svom sino-french space mission to observe the early afterglow of gamma-ray bursts in the 0.2-10 kev energy range and localize them within 2 arcmin. for a total mass of 42 kg and a total power of 60 w, this instrument is composed of an optical system, a structural carbon fiber tube, a radiator, a camera and a data processing unit. the camera consists of a focal plane assembly with a detector ceramic board and thermoelectric coolers, a front-end electronics, a calibration wheel and a support structure. spectral characterization tests of the focal plane assembly based on a 256 × 256 pixel pnccd spectroscopic imager were performed in the laboratory, at the metrology beamline of the soleil synchrotron and at the panter x-ray facility. energy resolution of 79 ev fwhm at 1.5 kev was demonstrated in the nominal flight configuration (-65 °c, real time event selection by the camera). the paper presents the design, the realization and the performance of the flight camera of mxt. it focuses on the technical challenges, the original implemented solutions and the custom-made setups for the validation tests. | design and performance of the camera of the micro-channel x-ray telescope on-board the svom mission |
we investigate the afterglow of grb 140713a, a gamma-ray burst (grb) that was detected and relatively well sampled at x-ray and radio wavelengths, but was not present at optical and near-infrared wavelengths, despite searches to deep limits. we present the emission spectrum of the likely host galaxy at z = 0.935 ruling out a high-redshift explanation for the absence of the optical flux detection. modelling the grb multiwavelength afterglow using the radiative transfer hydrodynamics code boxfit provides constraints on physical parameters of the grb jet and its environment, for instance a relatively wide jet opening angle and an electron energy distribution slope p below 2. most importantly, the model predicts an optical flux about two orders of magnitude above the observed limits. we calculated that the required host extinction to explain the observed limits in the r, i, and z bands was a^hostv > 3.2 mag, equivalent to e(b - v)host > 1.0 mag. from the x-ray absorption we derive that the grb host extinction is a^hostv = 11.6^{+7.5}_{-5.3} mag, equivalent to e(b-v)^host = 3.7^{+2.4}_{-1.7} mag, which is consistent with the extinction required from our boxfit derived fluxes. we conclude that the origin of the optical darkness is a high level of extinction in the line of sight to the grb, most likely within the grb host galaxy. | detailed multiwavelength modelling of the dark grb 140713a and its host galaxy |
gamma-ray burst (grb) 111215a was bright at x-ray and radio frequencies, but not detected in the optical or near-infrared (nir) down to deep limits. we have observed the grb afterglow with the westerbork synthesis radio telescope and arcminute microkelvin imager at radio frequencies, with the william herschel telescope and nordic optical telescope in the nir/optical, and with the chandra x-ray observatory. we have combined our data with the swift x-ray telescope monitoring, and radio and millimetre observations from the literature to perform broad-band modelling, and determined the macro- and microphysical parameters of the grb blast wave. by combining the broad-band modelling results with our nir upper limits we have put constraints on the extinction in the host galaxy. this is consistent with the optical extinction we have derived from the excess x-ray absorption, and higher than in other dark bursts for which similar modelling work has been performed. we also present deep imaging of the host galaxy with the keck i telescope, spitzer space telescope, and hubble space telescope (hst), which resulted in a well-constrained photometric redshift, giving credence to the tentative spectroscopic redshift we obtained with the keck ii telescope, and estimates for the stellar mass and star formation rate of the host. finally, our high-resolution hst images of the host galaxy show that the grb afterglow position is offset from the brightest regions of the host galaxy, in contrast to studies of optically bright grbs. | detailed afterglow modelling and host galaxy properties of the dark grb 111215a |
we develop a model of early gamma-ray burst (grb) afterglows with dominant x-ray contribution from the reverse shock (rs) propagating in highly relativistic (lorentz factor γw ∼ 106) magnetized wind of a long-lasting central engine. the model reproduces, in a fairly natural way, the overall trends and yet allows for variations in the temporal and spectral evolution of early optical and x-ray afterglows. the high energy and the optical synchrotron emission from the rs particles occurs in the fast cooling regime; the resulting synchrotron power ls is a large fraction of the wind luminosity, {l}s≈ {l}w/\sqrt{1+{σ }w} (lw and σw are wind power and magnetization). thus, plateaus—parts of afterglow light curves that show slowly decreasing spectral power—are a natural consequence of the rs emission. contribution from the forward shock (fs) is negligible in the x-rays, but in the optical both fs and rs contribute similarly: fs optical emission is in the slow cooling regime, producing smooth components, while rs optical emission is in the fast cooling regime, and thus can both produce optical plateaus and account for fast optical variability correlated with the x-rays, e.g., due to changes in the wind properties. we discuss how the rs emission in the x-rays and combined fs and rs emission in the optical can explain many puzzling properties of early grb afterglows. | early grb afterglows from reverse shocks in ultra-relativistic, long-lasting winds |
in order to explain rapid light-curve variability without invoking a variable source, several authors have proposed `minijets' that move relativistically relative to the main flow of the jet. here, we consider the possibility that these minijets, instead of being isotropically distributed in the comoving frame of the jet, form primarily perpendicular to the direction of the flow, as the jet dissipates its energy at a large emission radius. this yields two robust features. first, the emission is significantly delayed compared with the isotropic case. this delay allows for the peak of the afterglow emission to appear while the source is still active, in contrast to the simplest isotropic model. secondly, the flux decline after the source turns off is steeper than the isotropic case. we find that these two features are realized in gamma-ray bursts (grbs). (1) the peak of most gev light curves (ascribed to the external shock) appears during the prompt emission phase. (2) many x-ray light curves exhibit a period of steep decay, which is faster than that predicted by the standard isotropic case. the gamma-ray generation mechanism in grbs, and possibly in other relativistic flows, may therefore be anisotropic. | an anisotropic minijets model for the grb prompt emission |
we present a correlation between the average temporal decay ({α }{{x},{avg},\gt 200{{s}}}) and early-time luminosity ({l}{{x},200{{s}}}) of x-ray afterglows of gamma-ray bursts as observed by the swift x-ray telescope. both quantities are measured relative to a rest-frame time of 200 s after the γ-ray trigger. the luminosity-average decay correlation does not depend on specific temporal behavior and contains one scale-independent quantity minimizing the role of selection effects. this is a complementary correlation to that discovered by oates et al. in the optical light curves observed by the swift ultraviolet optical telescope. the correlation indicates that, on average, more luminous x-ray afterglows decay faster than less luminous ones, indicating some relative mechanism for energy dissipation. the x-ray and optical correlations are entirely consistent once corrections are applied and contamination is removed. we explore the possible biases introduced by different light-curve morphologies and observational selection effects, and how either geometrical effects or intrinsic properties of the central engine and jet could explain the observed correlation. | a correlation between the intrinsic brightness and average decay rate of gamma-ray burst x-ray afterglow light curves |
we present a wide data set of gamma-ray, x-ray, uv/opt/ir (uvoir), and radio observations of the swift grb100814a. at the end of the slow decline phase of the x-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. the optical rebrightening also shows chromatic evolution. such a puzzling behaviour cannot be explained by a single component model. we discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of grb100814 the best. | the optical rebrightening of grb100814a: an interplay of forward and reverse shocks? |
the multiwavelength observations of grb afterglows, together with some high-performance particle-in-cell simulations, hint that the magnetic field may decay behind the shock front. in this work, we develop a numerical code to calculate the evolution of the accelerated electron distribution, their synchrotron and inverse-compton (ic) spectra and accordingly the light curves (lcs) under the assumption of decaying microturbulence (dm) downstream of the shock, $\epsilon_b(t_p')\propto t_p'^{\alpha_t}$ with $t_p'$ the fluid proper time since injection. we find: (1) the synchrotron spectrum in the dm model is similar to that in the homogeneous turbulence (ht) model with very low magnetic field strength. however, the difference in the ic spectral component is relatively more obvious between them, due to the significant change of the postshock electron energy distribution with dm. (2) if the magnetic field decay faster, there are less electrons cool fast, and the ic spectral component becomes weaker. (3) the lcs in the dm model decay steeper than in the ht model, and the spectral evolution and the lcs in the dm model is similar to the ht model where the magnetic field energy fraction decreases with observer time, $\epsilon_b(t) \propto t^{5\alpha_t /8}$. (4) the dm model can naturally produce a significant ic spectral component in tev energy range, but due to the klein-nishina suppression the ic power cannot be far larger than the synchrotron power. we apply the dm model to describe the afterglow data of grb 190114c and find the magnetic field decay exponent $\alpha_t\sim -0.4$ and the electron spectral index $p\sim2.4$. future tev observations of the ic emission from grb afterglows will further help to probe the poorly known microphysics of relativistic shocks. | numerical study of synchrotron and inverse-compton radiation from gamma-ray burst afterglows with decaying microturbulence |
the jet composition of gamma-ray bursts (grbs), as well as how efficiently the jet converts its energy to radiation, are long-standing problems in grb physics. here, we reported a comprehensive temporal and spectral analysis of the tev-emitting bright grb 190114c. its high fluence ($\sim$ 4.436$\times$10$^{-4}$ erg cm$^{-2}$) allows us to conduct the time-resolved spectral analysis in great detail and study their variations down to a very short time-scale ($\sim$0.1 s) while preserving a high significance. its prompt emission consists of three well-separated pulses. the first two main pulses ($p_1$ and $p_2$) exhibit independently strong thermal components, and starting from the third pulse ($p_3$) and extending to the entire afterglow, the spectra are all non-thermal, the synchrotron plus compton up-scattering model well interpret the observation. by combining the thermal ($p_1$ and $p_2$) and the non-thermal ($p_3$) observations based on two different scenarios (global and pulse properties) and following the method described in \cite{zhang2021}, we measure the fireball parameters and grb radiative efficiency with little uncertainties for this grb. a relevantly high grb radiative efficiency is obtained based on both the global and pulse properties, suggesting that if grbs are powered by fireballs, the efficiency can be high sometimes. more interestingly, though the observed parameters are individually different (e.g., the amount of mass loading $m$), the radiative efficiency obtained from $p_1$ ($\eta_\gamma=36.0\pm6.5\%$) and $p_2$ ($\eta_\gamma=41.1\pm1.9\%$) is roughly the same, which implies that the central engine of the same grb has some common properties. | grb 190114c: fireball energy budget and radiative efficiency revisited |
in this paper, we compute and analyse synthetic radio images of gamma-ray bursts (grbs) and kilonova afterglows. for modelling the former, we consider grb170817a-inspired set of parameters, while for the latter, we employ ejecta profiles from numerical-relativity simulations. we find that the kilonova afterglow sky map has a doughnut-like structure at early times that becomes more ring-like at late times. this is caused by the fact that the synchrotron emission from electrons following maxwellian distribution function dominates the early beamed emission, while emissions from electrons following power-law distribution is important at late times. for an on-axis observer, the image flux centroid moves on the image plane, initially away from the observer. the image sizes, we find, are the largest for equal mass merger simulations with the soft equation of state. the presence of a kilonova afterglow affects the properties inferred from the source sky map, even if the grb afterglow dominates the total flux density. the main effect is the reduction of the mean apparent velocity of the source, and an increase in the source size. however, this effect becomes negligible around the light curve peak of the grb afterglow. for a far off-axis observer, neglecting the presence of the kilonova afterglow may lead to systematic errors in the inference of grb properties from the sky map observations. notably, at the observing angle inferred for grb170817a, the presence of kilonova afterglow would affect the sky map properties, only at very late times $t\gtrsim 1500\,$ d. | synthetic radio images of structured grb and kilonova afterglows |
we compare the isotropic equivalent 15-2000 kev γ-ray energy, eγ , emitted by a sample of 91 swift gamma-ray bursts with known redshifts, with the isotropic equivalent fireball energy, e fb, as estimated within the fireball model framework from x-ray afterglow observations of these bursts. the uncertainty in eγ , which spans the range of ∼1051 to ∼1053.5 erg, is ≈25% on average, due mainly to the extrapolation from the bat detector band to the 15-2000 kev band. the uncertainty in e fb is approximately a factor of 2, due mainly to the x-ray measurements’ scatter. we find eγand e fb to be tightly correlated. the average(std) of {η }γ 11 {hr}\equiv {{log}}10({e}γ /(3{\varepsilon }{{e}}{e}{{fb}}11 {hr})) are -0.34(0.60), and the upper limit on the intrinsic spread of ηγis approximately 0.5 ({\varepsilon }{{e}} is the fraction of energy carried by electrons and {e}{{fb}}x {hr} is inferred from the x-ray flux at x hours). {e}{{fb}}3 {hr} and {e}{{fb}}11 {hr} are similar, with an average(std) of {{log}}10({e}{{fb}}3 {hr}/{e}{{fb}}11 {hr}) of 0.04(0.28). the small variance of ηγimplies that burst-to-burst variations in {\varepsilon }{{e}} and in the efficiency of fireball energy conversion to γ-rays are small, and suggests that both are of order unity. the small variance of ηγand the similarity of {e}{{fb}}3 {hr} and {e}{{fb}}11 {hr} further imply that deviations from a simple fireball model description, if present, are small. this puts stringent constraints on models incorporating such modifications (due e.g., to radiative losses, energy injection, off-axis viewing). | the energy budget of grbs based on a large sample of prompt and afterglow observations |
grb 090618 is a bright gamma-ray burst (grb) with multiple pulses. it shows evidence of thermal emission in the initial pulses as well as in the early afterglow phase. because high-resolution spectral data from the swift/x-ray telescope (xrt) are available for the early afterglow, we investigate the shape and evolution of the thermal component in this phase using data from the swift/burst alert telescope (bat), the swift/xrt, and the fermi/gamma-ray burst monitor detectors. an independent fit to the bat and xrt data reveals two correlated blackbodies with monotonically decreasing temperatures. hence, we investigated the combined data with a model consisting of two blackbodies and a power law (2bbpl), a model suggested for several bright grbs. we elicit the following interesting features of the 2bbpl model: (1) the same model is applicable from the peak of the last pulse in the prompt emission to the afterglow emission, (2) the ratio of temperatures and the fluxes of the two blackbodies remains constant throughout the observations, (3) the blackbody temperatures and fluxes show a monotonic decrease with time, with the bb fluxes dropping about a factor of two faster than that of the power-law (pl) emission, and (4) attributing the blackbody emission to photospheric emissions, we find that the photospheric radii increase very slowly with time, and the lower-temperature blackbody shows a larger emitting radius than that of the higher-temperature blackbody. we find some evidence that the underlying shape of the nonthermal emission is a cutoff power law rather than a pl. we sketch a spine-sheath jet model to explain our observations. | discovery of smoothly evolving blackbodies in the early afterglow of grb 090618: evidence for a spine-sheath jet? |
we present predictions of centimeter and millimeter radio emission from reverse shocks (rss) in the early afterglows of gamma-ray bursts (grbs) with the goal of determining their detectability with current and future radio facilities. using a range of grb properties, such as peak optical brightness and time, isotropic equivalent gamma-ray energy, and redshift, we simulate radio light curves in a framework generalized for any circumburst medium structure and including a parameterization of the shell thickness regime that is more realistic than the simple assumption of thick- or thin-shell approximations. building on earlier work by mundell et al. and melandri et al. in which the typical frequency of the rs was suggested to lie at radio rather than optical wavelengths at early times, we show that the brightest and most distinct rs radio signatures are detectable up to 0.1-1 day after the burst, emphasizing the need for rapid radio follow-up. detection is easier for bursts with later optical peaks, high isotropic energies, lower circumburst medium densities, and at observing frequencies that are less prone to synchrotron self-absorption effects—typically above a few ghz. given recent detections of polarized prompt gamma-ray and optical rs emission, we suggest that detection of polarized radio/millimeter emission will unambiguously confirm the presence of low-frequency rss at early time. | radio flares from gamma-ray bursts |
we have performed a systematic study of gamma-ray bursts (grbs), which have various values in the peak energy of the νfν spectrum of prompt emission, epeak, observed by the swift/burst alert telescope (bat) and fermi/gamma-ray burst monitor, investigating their prompt and x-ray afterglow emissions. we cataloged long-lasting grbs observed by swift between 2004 december and 2014 february in three categories according to the classification by sakamoto et al.: x-ray flashes (xrfs), x-ray-rich grbs (xrrs), and classical grbs (c-grbs). we then derived ${e}_{\mathrm{peak}}^{\mathrm{obs}}$ , as well as ${e}_{\mathrm{peak}}^{\mathrm{src}}$ if viable, of the swift spectra of their prompt emission. we also analyzed their x-ray afterglows and found that grb events with a lower ${e}_{\mathrm{peak}}^{\mathrm{src}}$ , i.e., softer grbs, are fainter in 0.3-10 kev x-ray luminosity and decay more slowly than harder grbs. the intrinsic event rates of the xrfs, xrrs, and c-grbs were calculated using the swift/bat trigger algorithm. those of the xrrs and xrfs are larger than that of the c-grbs. if we assume that the observational diversity of epeak is explained using the off-axis model, these results yield a jet half-opening angle of δθ ∼ 0°3, and a variance of the observing angles θobs ≲ 0°6. this implies that this tiny variance would be responsible for the epeak diversity observed by swift/bat, which is unrealistic. therefore, we conclude that the epeak diversity is not explained with the off-axis model, but is likely to originate from some intrinsic properties of the jets. | systematic study of the peak energy of broadband gamma-ray burst spectra |
jet precessions are widely involved in astrophysical phenomena from galaxies to x-ray binaries and gamma-ray bursts (grbs). polarization presents a unique probe of the magnetic fields in grb jets. the precession of grb relativistic jets will change the geometry within the observable emitting region of the jet, which can potentially affect the polarization of the afterglow. in this paper, we take into account jet precession to study the polarization evolution and corresponding light curves in grb early optical afterglows with ordered and random magnetic field geometries. we find that the jet precession in long-lived engines can significantly reduce the polarization degree (pd) regardless of the magnetic field structure. the strongest pd attenuation is found when the line of sight is aligned with the precession axis. our results show that jet precession can provide new insight into the low pd measured in the early optical afterglows of grbs. | polarization in early optical afterglows of gamma-ray bursts driven by precessing jets |
the angular distribution of gamma-ray burst (grb) jets is not yet clear. the observed luminosity of grb 170817a is the lowest among all known short grbs, which is best explained by the fact that our line of sight is outside of the jet opening angle, θ obs > θj , where θ obs is the angle between our line of sight and the jet axis. as inferred by gravitational wave observations, as well as radio and x-ray afterglow modeling of grb 170817a, it is likely that θ obs ∼ 20° - 28°. in this work, we quantitatively consider two scenarios of angular energy distribution of grb ejecta: a top-hat jet and a structured jet with a power law index s. for the top-hat jet model, we get a large θj(e.g., θj> 10°), a rather high local (i.e., z < 0.01) short grb rate ∼8-15×103, gpc-3, yr-1 (estimated to be 90∼1850 gpc-3, yr-1 in fong et al.) and an extremely high e peak,0 (on-axis, rest-frame) > 7.5 × 104, kev (∼500, kev for a typical short grb). for the structured jet model, we use θ obs to give limits on s and θj for typical on-axis luminosity of a short grb (e.g., 1049 erg s-1 ∼ 1051 erg s-1), and a low on-axis luminosity case (e.g., 1049 erg s-1) gives more reasonable values of s. the structured jet model is more feasible for grb 170817a than the top-hat jet model due to the rather high local short grb rate, and the extremely high on-axis e peak,0 almost rules out the top-hat jet model. grb 170817a is likely a low on-axis luminosity grb (1049 erg s-1) with a structured jet. | grb 170817a: a short grb seen off-axis |
as we further our studies on gamma-ray bursts (grbs), via both theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. as grbs are transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by grbs should present some degree of polarization throughout the evolution of the burst. whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short grbs, it is paramount that the groundwork is laid for the day we have abundant data. in this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile n(r) ∝ r -k . we present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the grb literature for four degrees of stratification k = 0, 1, 1.5, and 2 and two magnetic field configurations with high extreme anisotropy. we apply this model and predict polarization from a set of grbs exhibiting off-axis afterglow emission. in particular, for grb 170817a, we use the available polarimetric upper limits to rule out the possibility of an extremely anisotropic configuration for the magnetic field. | afterglow polarization from off-axis gamma-ray burst jets |
tev emissions from γ-ray bursts are very important to study their origin and the radiation mechanisms in detail. recent observations of tev photons in some of the grbs are challenging to be explained by the traditional synchrotron radiation mechanism. in this work, we present the results of a detailed investigation of the prompt and afterglow emissions of recently discovered tev grbs (grb 180720b, grb 190114c, and grb 190829a) based on the publicly available prompt and afterglow data including 10.4m gtc and 1.3m dfot telescopes observations of the first hess and magic bursts, respectively. timeresolved spectroscopy of prompt emission of grb 180720b and grb 190114c shows an intensity tracking nature of peak energy. in the case of grb 190829a, peak energy evolution shows a hard to soft tracking trend followed by a very soft and chaotic trend. grb 190829a is a peculiar intermediate luminous two episodic burst with first emission episode outlier to amati correlation. we analyzed the late time fermi-lat emission that encapsulates the h.e.s.s. and magic observations. some of the lat photons are likely to be associated with these grbs and they could have an inverse compton radiation mechanism. | observational properties of tev detected grb 180720b, grb 190114c and grb 190829a |
previous studies have shown that the incidence rate of intervening strong mg ii absorbers towards gamma-ray bursts (grbs) were a factor of 2-4 higher than towards quasars. exploring the similar sized and uniformly selected legacy data sets xq-100 and xsgrb, each consisting of 100 quasar and 81 grb afterglow spectra obtained with a single instrument (vlt/x-shooter), we demonstrate that there is no disagreement in the number density of strong mg ii absorbers with rest-frame equivalent widths w_rλ2796>1 å towardsgrbs and quasars in the redshift range 0.1 ≲ z ≲ 5. with large and similar sample sizes, and path length coverages of δz = 57.8 and 254.4 for grbs and quasars, respectively, the incidences of intervening absorbers are consistent within 1σ uncertainty levels at all redshifts. for absorbers at z < 2.3, the incidence towards grbs is a factor of 1.5 ± 0.4 higher than the expected number of strong mg ii absorbers in sloan digital sky survey (sdss) quasar spectra, while for quasar absorbers observed with x-shooter we find an excess factor of 1.4 ± 0.2 relative to sdss quasars. conversely, the incidence rates agree at all redshifts with reported high-spectral-resolution quasar data, and no excess is found. the only remaining discrepancy in incidences is between sdss mg ii catalogues and high-spectral-resolution studies. the rest-frame equivalent-width distribution also agrees to within 1σ uncertainty levels between the grb and quasar samples. intervening strong mg ii absorbers towards grbs are therefore neither unusually frequent, nor unusually strong. based on observations collected at the european southern observatory, paranal, chile, program id: 098.a-0055, 097.a-0036, 096.a-0079, 095.b-0811(b), 095.a-0045, 094.a-0134, 093.a-0069, 092.a-0124, 0091.c-0934, 090.a-0088, 089.a-0067, 088.a-0051, 087.a-0055, 086.a-0073, 085.a-0009 and 084.a-0260. xq-100: 189.a-0424. | solving the conundrum of intervening strong mg ii absorbers towards gamma-ray bursts and quasars |
it has recently become clear that in both short and long gamma-ray bursts (grbs) it coexists a sequence of different events, each characterized by specific physical processes and corresponding values of the lorentz gamma factors. the ultra-relativistic prompt emission (upe) phase, with lorentz factor $\gamma\leq10^4$, is followed by a mildly relativistic plateau-afterglow phase with $\gamma\lesssim2$. the gev radiation, with $\gamma\lesssim50$, coexists with the above two phases. it is shown that: a) the gev radiation originates at the onset of the formation of a black hole (bh), b) its luminosity follows specific power-law dependence when measured in the rest frame of the source with a decay index $\gamma=-1.29\pm0.06$ in the case of the short grbs, and $\gamma=-1.20\pm0.04$ in the case of the long grbs, c) these energetics requirements are used to derive the mass and spin of the bh originating this extended gev emission. we present these conceptual results for the case of short grbs in this article and give the extended analysis for long grbs in a companion article. a direct astrophysical application of these results is that the merger of binary neutron stars leading to bh formation emits gev radiation: the gev emission is a necessary and sufficient condition to indicate the creation of a bh in s-grbs. | on the short grb gev emission from a kerr black hole |
we investigate the expected radio emission from the reverse shock of short grbs, using the fitted afterglow parameters. in light of recent results suggesting that in some cases the radio afterglow is due to emission from the reverse shock, we examine the extent to which this component is detectable for short grbs. in some grbs, the standard synchrotron shock model predicts detectable radio emission from the reverse shock when none was seen. many physical parameters play a role in these estimates, and our results highlight the need to explore the fundamental processes involved in grb particle acceleration and emission more deeply. however, with a more rapid follow-up, we can test our standard model of grbs, which predicts an early, radio bright reverse shock in many cases. | reverse shock emission from short grbs |
the rich and complex data obtained from multi-wavelength observations of shb170817a, the short hard gamma ray burst (shb) associated with gw170817 --the first neutron stars merger event detected in gravitational waves (gws)-- are analyzed in the framework of the cannonball model of shbs. in this model a highly relativistic jet is launched by fall back matter on the nascent neutron star (or black hole) into a surrounding glory (light from the surrounding wind nebula of the binary neutron stars) which was present already before the merger. the shb was produced by inverse compton scattering of glory photons by the jet, which was viewed far off-axis. the fading glory, which produced the initial uvoir afterglow, was powered by a neutron star remnant. it was overtaken by a late time x-ray, uvoir and radio afterglow produced by synchrotron radiation from the decelerating jet in the interstellar medium of the host galaxy. if the radio afterglow of shb170817a was indeed produced by the jet, it should display a superluminal motion relative to the shb location, still detectable in vla and vlbi radio observations. | cannonball model diagnosis of the short gamma ray burst 170817a |
gamma-ray burst (grb) 120729a was detected by swift/bat and fermi/gbm, and then rapidly observed by swift/xrt, swift/uvot, and ground-based telescopes. it had a single long and smooth γ-ray emission pulse, which extends continuously to the x-rays. we report lick/kait observations of the source, and make temporal and spectral joint fits of the multiwavelength light curves of grb 120729a. it exhibits achromatic light-curve behavior, consistent with the predictions of the external shock model. the light curves are decomposed into four typical phases: onset bump (phase i), normal decay (phase ii), shallow decay (phase iii), and post-jet break (phase iv). the spectral energy distribution (sed) evolves from prompt γ-ray emission to the afterglow with a photon index from γ γ= 1.36 to γ ≈ 1.75. there is no obvious evolution of the sed during the afterglow. the multiwavelength light curves from γ-ray to optical can be well modeled with an external shock by considering energy injection, and a time-dependent microphysics model with {ɛ }b\propto {t}{α b} for the emission at early times, t< {t}0+157 {{s}}. therefore, we conclude that both the prompt γ-ray emission and afterglow of grb 120729a have the same external shock physical origin. our model indicates that the ɛbevolution can be described as a broken power-law function with αb,1 = 0.18 ± 0.04 and αb,2 = 0.84 ± 0.04. we also systematically investigate single-pulse grbs in the swift era, finding that only a small fraction of grbs (grbs 120729a, 051111, and 070318) are likely to originate from an external shock for both the prompt γ-ray emission and afterglow. | grb 120729a: external shock origin for both the prompt gamma-ray emission and afterglow |
gamma-ray burst (grb) 150910a was detected by swift/burst alert telescope (bat), and then rapidly observed by swift/xrt, swift/ultraviolet-optical telescope, and ground-based telescopes. we report lick observatory spectroscopic and photometric observations of grb 150910a, and we investigate the physical origins of both the optical and x-ray afterglows, incorporating data obtained with bat and xrt. the light curves show that the jet-emission episode lasts ∼360 s with a sharp pulse from bat to xrt (episode i). in episode ii, the optical emission has a smooth onset bump followed by a normal decay (αr,2 ≈ -1.36), as predicted in the standard external shock model, while the x-ray emission exhibits a plateau (αx,1 ≈ -0.36) followed by a steep decay (αx,2 ≈ -2.12). the light curves show obvious chromatic behavior with an excess in the x-ray flux. our results suggest that grb 150910a is an unusual grb driven by a newly born magnetar with its extremely energetic magnetic dipole (md) wind in episode ii, which overwhelmingly dominates the observed early x-ray plateau. the radiative efficiency of the jet prompt emission is ηγ ≈ 11%. the md wind emission was detected in both the bat and xrt bands, making it the brightest among the current sample of md winds seen by xrt. we infer the initial spin period (p0) and the surface polar cap magnetic field strength (bp) of the magnetar as 1.02 × 1015 g ≤ bp ≤ 1.80 × 1015 g and 1 ms ≤ p0v ≤ 1.77 ms, and the radiative efficiency of the wind is ηw ≥ 32%. | early optical observations of grb 150910a: bright jet optical afterglow and x-ray dipole radiation from a magnetar central engine |
we present the second swift ultra-violet/optical telescope (uvot) gamma-ray burst (grb) afterglow catalog, greatly expanding on the first swift uvot grb afterglow catalog. the second catalog is constructed from a database containing over 120,000 independent uvot observations of 538 grbs first detected by swift, the high energy transient explorer 2 (hete2), the international gamma-ray astrophysics laboratory (integral), the interplanetary network (ipn), fermi, and astro-rivelatore gamma a immagini leggero (agile). the catalog covers grbs discovered from 2005 january 17 to 2010 december 25. using photometric information in three uv bands, three optical bands, and a “white” or open filter, the data are optimally coadded to maximize the number of detections and normalized to one band to provide a detailed light curve. the catalog provides positional, temporal, and photometric information for each burst, as well as swift burst alert telescope and x-ray telescope (xrt) grb parameters. temporal slopes are provided for each uvot filter. the temporal slope per filter of almost half the grbs are fit with a single power law, but one to three breaks are required in the remaining bursts. morphological comparisons with the x-ray reveal that ∼ 75 % of the uvot light curves are similar to one of the four morphologies identified by evans et al. (2009). the remaining ∼ 25 % have a newly identified morphology. for many bursts, redshift- and extinction-corrected uv/optical spectral slopes are also provided at 2 × 103, 2 × 104, and 2 × 105 s. | a large catalog of homogeneous ultra-violet/optical grb afterglows: temporal and spectral evolution |
recently, delaunay et al. (2016) discovered a gamma-ray transient, swift j0644.5-5111, associated with the fast radio burst (frb) 131104. they also reported follow-up broadband observations beginning two days after the frb and provided upper limits on a putative afterglow of this transient. in this paper, we show that if such a transient drives a relativistic shock as in a cosmological gamma-ray burst (grb), these upper limits are consistent with an environment of which density is much less than that of an interstellar medium but typical for the outskirts' density of a galaxy when the typical values of three microphysical parameters of the shock are taken. this appears to be inconsistent with the catastrophic event models in which the central engine of swift j0644.5-5111 is surrounded by an interstellar medium, but together with the properties of the gamma-ray transient, favors the binary neutron star merger origin. we further constrain the physical parameters of the postmerger object by assuming that swift j0644.5-5111 results from internal dissipation of a spinning-down pulsar wind, and we find that the postmerger object is an ultra-strongly magnetized, very rapidly rotating pulsar. this merger event should have given birth to a gravitational wave burst, an frb, and a short grb or an extended x-ray/gamma-ray emission if a relativistic jet of the grb is missed. such "triplets" would be testable in the near future with the advanced ligo and virgo gravitational-wave observatories. | could frb 131104 originate from the merger of binary neutron stars? |
we derive basic analytical results for the timing and decay of the gamma-ray burst (grb) counterpart and delayed afterglow light curves for a brief emission episode from a relativistic surface endowed with angular structure, consisting of a uniform core of size ${\theta }_{c}$ (lorentz factor ${{\rm{\gamma }}}_{c}$ and surface emissivity ${i}_{\nu ^{\prime} }^{{\prime} }$ are angle independent) and an axially symmetric power-law envelope ( ${\rm{\gamma }}\sim {\theta }^{-g}$ ). in this "large-angle emission" model, radiation produced during the prompt emission phase (grb) at angles $\theta \gt {\theta }_{c}$ arrives at the observer well after the burst (delayed emission). the dynamical time range of the very fast decaying grb "tail" and of the flat afterglow "plateau" and the morphology of the grb counterpart/afterglow are all determined by two parameters: the core's parameter ${{\rm{\gamma }}}_{c}{\theta }_{c}$ and the envelope's lorentz factor index g, leading to three types of light curves that display three post-grb phases (type 1: tail, plateau/slow decay, post-plateau/normal decay), two post-grb phases (type 2: tail and fast decay), or just one (type 3: normal decay). we show how x-ray light-curve features can be used to determine core and envelope dynamical and spectral parameters. testing of the large-angle emission model is done using the swift/xrt x-ray emission of two afterglows of type 1 (grb 060607a, grb 061121), one of type 2 (grb 061110a), and one of type 3 (grb 061007). we find that the x-ray afterglows with plateaus require an envelope lorentz factor ${\rm{\gamma }}\sim {\theta }^{-2}$ and a comoving-frame emissivity ${i}_{\nu ^{\prime} }^{{\prime} }\sim {\theta }^{2}$ ; thus, for a typical afterglow spectrum ${f}_{\nu }\sim {\nu }^{-1}$ , the lab-frame energy release is uniform over the emitting surface. | x-ray afterglows from the gamma-ray burst "large-angle" emission |
the observed early x-ray plateau in the afterglow lightcurves of some gamma-ray bursts (grbs) is attributed to the dipole radiations (drs) of a newborn magnetar. a quasi-periodic oscillation (qpo) signal in the plateau would be strong evidence of the magnetar precession motion. by making a time-frequency domain analysis for the x-ray afterglow lightcurve of grb 180620a, we find a qpo signal of ~650 s in its early x-ray plateau. we fit the lightcurve with a magnetar precession model by adopting the markov chain monte carlo algorithm. the observed lightcurve and the qpo signal are well represented with our model. the derived magnetic field strength of the magnetar is $b_{\rm p}= (1.02^{+0.59}_{-0.61})\times 10^{15}$ g. it rapidly spins down with angular velocity evolving as ωs ∝ (1 + t/τsd)-0.96, where τsd = 9430 s. its precession velocity evolution is even faster than ωs, i.e. ωp ∝ (1 + t/τp)-2.18 ± 0.11, where τp = 2239 ± 206 s. the inferred braking index is n = 2.04. we argue that the extra energy loss via the magnetospheric processes results in its rapid spin-down, low braking index, and strong precession motion of the magnetar. | early evolution of a newborn magnetar with strong precession motion in grb 180620a |
gamma-ray bursts (grbs) can be divided into three subclasses: x-ray flash (xrf), x-ray rich (xrr), and classical grb (c-grb). an x-ray flare is the rebrightening emission shown in the early x-ray afterglow of some grbs. in this paper, we comprehensively examine the x-ray flare properties among xrf, xrr, and c-grb subclasses. we utilize the xrf, xrr, and c-grb subclass samples obtained from the swift-bat3 catalog, and the x-ray flare observational properties are collected from falcone et al., chincarini et al., and yi et al. we find that xrfs and xrrs have more bright x-ray flares than c-grbs. the ratio of the x-ray flare fluence to the prompt emission fluence has different distributions between xrf and c-grb subclasses. the linear correlation between the duration and the peak time of the x-ray flares is also different between xrf and c-grb subclasses. we are inclined to identify the grbs with the bright x-ray flares as xrfs or xrrs. we discuss some issues that are related to the xrf/xrr/c-grb classification. we also caution the selection effects and the instrument bias in our investigation. large samples are required in the future to further confirm our results. | grb x-ray flare properties among different grb subclasses |
the experimental study of the dynamics of neutral electron-positron beams is an emerging area of research, enabled by the recent results on the generation of this exotic state of matter in the laboratory. electron-positron beams and plasmas are believed to play a major role in the dynamics of extreme astrophysical objects such as supermassive black holes and pulsars. for instance, they are believed to be the main constituents of a large number of astrophysical jets, and they have been proposed to significantly contribute to the emission of gamma-ray bursts and their afterglow. however, despite extensive numerical modelling and indirect astrophysical observations, a detailed experimental characterisation of the dynamics of these objects is still at its infancy. here, we will report on some of the general features of experiments studying the dynamics of electron-positron beams in a fully laser-driven setup. | general features of experiments on the dynamics of laser-driven electron-positron beams |
instruments, such as the rotse, tortora, pi of the sky, master-net, and others have recorded single-band optical flux measurements of gamma-ray bursts starting as early as ~ 10 s after gamma-ray trigger. the earliest measurements of optical spectral shape have been made only much later, typically on hour time-scales, never starting less than a minute after trigger, until now. beginning only 58 s after theswift bat triggered on grb201015a, we observed a sharp rise in optical flux to a peak, followed by a power law temporal decay, ∝ t-0.81 ± 0.03. flux was measured simultaneously in three optical bands, g', r', and i', using our burst simultaneous three-channel imager on the nazarbayev university transient telescope at assy-turgen astrophysical observatory telescope. our data during the decay show strong colour evolution from red to blue, with a change in the optical log slope of +0.72 ± 0.14; during this time the x-ray log slope remained constant. we did not find evidence for a two-component jet structure or a transition from reverse to forward shock or a prompt emission component that would explain this change in slope. we find that the majority of the optical spectral slope evolution is consistent with a monotonic decay of extinction, evidence of dust destruction. assuming a constant source spectral slope and an small magellanic cloud-like extinction curve, we derive a change in the local extinction $a_\mathrm{v}^\mathrm{local}$ from ~0.8 mag to 0.3 mag in ~2500 s. this work shows that significant information about the early emission phase is being missed without such early observations with simultaneous multiband instruments. | evolution of the afterglow optical spectral shape of grb 201015a in the first hour: evidence for dust destruction |
the recent radio observations of a superluminal radio afterglow following gamma-ray burst (grb) 179817a are interpreted in terms of a jet impacting a baryonic cloak, which is presumably the material caught at the front of the jet as the latter emerges from a denser ejected material. assuming that we, the observers, are located at a viewing angle of ∼0.2 radians from the emitting material (perhaps slightly more from jet axis), we suggest that the lorentz factor of the jet is ≲20 at the time of the prompt emission, and that, as suggested previously, it is accelerated to much higher values before finally decelerating during the afterglow phase. a less extreme example of a short grb being observed off-axis may have been grb 150101b. a feature of grbs viewed from large offset angles is a large afterglow isotropic equivalent energy as compared to prompt emission, as predicted, and this is born out by the observations of these two grb. it is also shown that the prompt emission of grb 170817a, if seen way off-axis (θ ≫ 1/γ), could not be made by internal shocks in the baryonic material that powers the afterglow. | short gamma-ray bursts viewed from far off-axis |
in this paper, we further investigate the relationship, reported by oates et al., between the optical/uv afterglow luminosity (measured at restframe 200 s) and average afterglow decay rate (measured from restframe 200 s onwards) of long duration gamma-ray bursts (grbs). we extend the analysis by examining the x-ray light curves, finding a consistent correlation. we therefore explore how the parameters of these correlations relate to the prompt emission phase and, using a monte carlo simulation, explore whether these correlations are consistent with predictions of the standard afterglow model. we find significant correlations between: log lo, 200 s and log lx, 200 s; αo, >200 s and αx, >200 s, consistent with simulations. the model also predicts relationships between log eiso and log l200 s; however, while we find such relationships in the observed sample, the slope of the linear regression is shallower than that simulated and inconsistent at ≳3σ. simulations also do not agree with correlations observed between log l200 s and α> 200 s, or loge_{iso} and α> 200 s. overall, these observed correlations are consistent with a common underlying physical mechanism producing grbs and their afterglows regardless of their detailed temporal behaviour. however, a basic afterglow model has difficulty explaining all the observed correlations. this leads us to briefly discuss alternative more complex models. | exploring the canonical behaviour of long gamma-ray bursts using an intrinsic multiwavelength afterglow correlation |
advances in our numerical and theoretical understanding of gamma-ray burst afterglow processes allow us to construct models capable of dealing with complex relativistic jet dynamics and non-thermal emission, that can be compared directly to data from instruments such as swift. because afterglow blast waves and power law spectra are intrinsically scale-invariant under changes of explosion energy and medium density, templates can be generated from large-scale hydrodynamics simulations. this allows for iterative template-based model fitting using the physical model parameters (quantifying the properties of the burster, emission and observer) directly as fit variables. here i review how such an approach to afterglow analysis works in practice, paying special attention to the underlying model assumptions, possibilities, caveats and limitations of this type of analysis. because some model parameters can be degenerate in certain regions of parameter space, or unconstrained if data in a limited number of a bands is available, a bayesian approach is a natural fit. the main features of the standard afterglow model are reviewed in detail. | simulation and physical model based gamma-ray burst afterglow analysis |
following the recently established "binary-driven hypernova" (bdhn) model, we here interpret grb 970828 in terms of the four episodes typical of such a model. the "episode 1," up to 40 s after the trigger time t 0, with a time varying thermal emission and a total energy of eiso,1st = 2.60 × 1053 erg, is interpreted as due to the onset of an hyper-critical accretion process onto a companion neutron star, triggered by the companion star, an feco core undergoing a sn explosion. the "episode 2," observed up t 0 + 90 s, is interpreted as a canonical gamma ray burst, with an energy of erg, a baryon load of b = 7× 10-3 and a bulk lorentz factor at transparency of g = 142.5. from this episode 2, we infer that the grb exploded in an environment with a large average particle density <n> ≈103 particles/cm3 and dense clouds characterized by typical dimensions of (4-8) × 1014 cm and δn/n ~ 10. the "episode 3" is identified from t 0 +90 s all the way up to 105-6 s: despite the paucity of the early x-ray data, typical in the batse, pre-swift era, we find extremely significant data points in the late x-ray afterglow emission of grb 970828, which corresponds to the ones observed in all bdhne sources. the "episode 4," related to the supernova emission, does not appear to be observable in this source, due to the presence of darkening from the large density of the grb environment, also inferred from the analysis of the episode 2. | induced gravitational collapse in the batse era: the case of grb 970828 |
advanced ligo and virgo (advligo/virgo) detectors reported the first binary neutron star merger candidate in the third observing run, s190425z, on 2019 april 25. a weak γ-ray excess was reported nearly coincidentally by the international gamma-ray astrophysics laboratory (integral) satellite, which accidentally covered the entire localization region of advligo/virgo. electromagnetic follow-up in longer wavelengths has not lead to the detection of any associated counterparts. here we combine the available information from gravitational wave measurements and upper limits of fluence from integral to show that the observations are completely consistent with a relativistic gaussian structured jet and a typical short duration gamma-ray burst (grb) being produced in the merger. we obtain posterior bounds on the on-axis isotropic equivalent energy of the associated grb under different prior distributions. this study demonstrates that even limited gravitational wave and electromagnetic information could be combined to produce valuable insights about outflows from mergers. future follow-ups may help constrain the jet structure further, especially if there is an orphan afterglow detection associated with the candidate. | on the energetics of a possible relativistic jet associated with the binary neutron star merger candidate s190425z |
linear polarization measurements in the optical band show polarization degrees of a few per cent at late times. recently, polarization at sub-per-cent level was also detected in radio by alma, opening the window for multiwavelength polarimetry and stressing the importance of properly modelling polarization in grb afterglows across the em spectrum. we introduce a numerical tool that can calculate the polarization from relativistically moving surfaces by discretizing them to small patches of uniform magnetic field, calculating the polarized emission from each cell assuming synchrotron radiation and summing it to obtain the total degree of polarization. we apply this tool to afterglow shocks with random magnetic fields confined to the shock plane, considering electron radiative cooling. we analyse the observed polarization curves in several wavelengths above the cooling frequency and below the minimal synchrotron frequency and point to the characteristic differences between them. we present a method to constrain the jet opening angle and the viewing angle within the context of our model. applying it to grb 021004, we obtain angles of ~10○ and ~8○ respectively and conclude that a non-negligible component of radial magnetic field is required to explain the ${\sim}1{{\ \rm per\ cent}}$ polarization level observed 3.5 days after the burst. | modelling the linear polarization of grb afterglows across the electromagnetic spectrum |
observations of gamma-ray bursts (grbs) at very high energy (vhe) offer a unique opportunity to investigate particle acceleration processes, magnetic fields and radiation fields in these events. very long baseline interferometry (vlbi) observations have been proven to be a powerful tool providing unique information on the source size of the grbs at mas scales, as well as their accurate positions and possible expansion speeds. this paper reports on the follow-up observations of grb 190114c, the first ever grb detected with high significance at tev photon energies by the magic telescope, conducted with the east asia vlbi network (eavn) at 22 ghz on three epochs, corresponding to 6, 15 and 32 days after the burst. the derived maps do not show any significant source above 5 sigma. the inferred upper limits on the grb 190114c flux density at 22 ghz are used here to constrain the allowable two-dimensional parameter space for the afterglow emission. we find that our limits are consistent with most afterglow parameter combinations proposed so far in the literature. this is the first effort for the eavn to search and monitor a radio transient in the target of opportunity mode. in addition to the useful constraints on grb 190114c radio emission, experience gained from these observations is very helpful for future routine operation of eavn transient program. | east asia vlbi network observations of the tev gamma-ray burst 190114c |
the spectral index s of high-energy particles diffusively accelerated in a non-magnetized relativistic shock, such as in a γ-ray burst afterglow, depends on the unknown angular diffusion function $\mathcal {d}$ , which itself depends on the particle distribution function f if acceleration is efficient. we develop a relaxation code to compute s and f for an arbitrary functional $\mathcal {d}$ that depends on f. a local $\mathcal {d}(f)$ dependence is motivated and shown, when rising (falling) upstream, to soften (harden) s with respect to the isotropic case, shift the angular distribution towards upstream (downstream) directions, and strengthen (weaken) the particle confinement to the shock; an opposite effect on s is found downstream. however, variations in s remain modest even when $\mathcal {d}$ is a strong function of f, so the standard, isotropic-diffusion results remain approximately applicable unless $\mathcal {d}$ is both highly anisotropic and not a local function of f. a mild, ∼0.1 softening of s, in both 2d and 3d, when $\mathcal {d}(f)$ rises sufficiently fast, may be realized in ab initio simulations. | diffusive acceleration in relativistic shocks: particle feedback |
the afterglow of grb 170817a has been detected for more than three years, but the origin of the multi-band afterglow light curves remains under debate. a classical top-hat jet model is faced with difficulties in producing a shallow rise of the afterglow light curves as observed (fν ∝ t0.8). here we reconsider the model of stratified ejecta with an energy profile of e(>γ β) = e0(γ β)-k as the origin of the afterglow light curves of the burst, where γ and β are the lorentz factor and speed of the ejecta, respectively. k is the power-law slope of the energy profile. we consider that the ejecta are collimated into jets. two kinds of jet evolutions are investigated, including a lateral-spreading jet and a non-lateral-spreading jet. we fit the multi-band afterglow light curves, including the x-ray data at one thousand days post-burst, and find that both the models of the spreading and non-spreading jets can fit the light curves well, but the observed angular size of the source and the apparent velocity of the flux centroid for the spreading jet model are beyond the observation limits, while the non-spreading jet model meets the observation limits. some of the best-fit parameters for the non-spreading jet model, such as the number density of the circumburst medium ~10-2 cm-3 and the total jet kinetic energy e ~ 4.8 × 1051 erg, also appear plausible. the best-fit slope of the jet energy profile is k ~ 7.1. our results suggest that the afterglow of grb 170817a may arise from the stratified jet and that the lateral spreading of the jet is not significant. | the afterglow emission from a stratified jet in grb 170817a |
the gravitational waves from the binary neutron star merger gw170817 were accompanied by a multiwavelength electromagnetic counterpart, which confirms the association of the merger with a short gamma-ray burst (sgrb). the afterglow observations implied that the event was accompanied by a narrow, ~5°, and powerful, ~1050 erg, jet. we study the propagation of a poynting flux-dominated jet within the merger ejecta (kinematic, neutrino-driven, and magnetorotational instability turbulence-driven) of a neutrino-radiation-grmhd simulation of two coalescing neutron stars. we find that the presence of a postmerger low-density/low-pressure polar cavity, which arose due to angular momentum conservation, is crucial to let the jet break out. at the same time the ejecta collimates the jet to a narrow opening angle. the collimated jet has a narrow opening angle of ~4°-7° and an energy of 1049-1050 erg, in line with the observations of gw170817 and other sgrbs. | the collimation of relativistic jets in post-neutron star binary merger simulations |
for an expanding spherical relativistic shock, we derive relations between the parameters of downstream emitting zone and the quantities measured by a distant observer. these relations are formulated in terms of dimensionless effective coefficients combined with self-evident dimensional estimates. our calculations take into account evolution of the shock's lorentz factor, geometrical delay due to the shock's front curvature, and angular dependence of lorentz boost for frequency and brightness. the relations are designed primarily for application in gamma-ray burst afterglow studies, although they may have a broader use. | relating quasi-stationary one zone emission models to expanding relativistic shocks |
x-ray flares in gamma-ray bursts (grbs) are believed to be generated by the late activities of central engine, and thus provide an useful tool to diagnose the properties of central objects. in this paper, we work on a grb x-ray flare sample whose bulk lorentz factors are constrained by two different methods and the jet opening angles are determined by the jet breaks in afterglow light curves. considering a hyperaccreting stellar-mass black hole (bh) as the central engine of grbs and the blandford & znajek process (bz) as the jet production mechanism, we constrain the parameters of central engine by using the x-ray flare data. we find that the bz mechanism is so powerful making it possible to interpret both grb prompt emissions and bright x-ray flares. the wind parameter (p) and accreted mass (md) fall into reasonable ranges. our result is also applied to grb 170817a. the late x-ray flare in grb 170817a, if it is true, might not be a bh origin. | constraining properties of grb central engines with x-ray flares |
observations of afterglows of gamma-ray bursts (grbs) suggest that post-shock magnetic fields are strongly amplified to about 100 times the shock-compressed value. the weibel instability appears to play an important role in generating the magnetic field. however, recent simulations of collisionless shocks in homogeneous plasmas show that the magnetic field generated by the weibel instability rapidly decays. there must be some density fluctuations in interstellar and circumstellar media. the density fluctuations are anisotropically compressed in the downstream region of relativistic shocks. in this paper, we study the weibel instability in electron-positron plasmas with spatially anisotropic density distributions by means of two-dimensional particle-in-cell simulations. we find that large magnetic fields are maintained for a longer time by the weibel instability driven by spatially anisotropic density structure. particles anisotropically escape from the high density region, so that a temperature anisotropy is generated and the weibel instability becomes unstable. our simulation results suggest that the weibel instability driven by an anisotropic density structure can generate sufficiently large magnetic fields and they can cover sufficiently large regions to explain the afterglow emission of grbs. | weibel instability driven by spatially anisotropic density structures |
here we present the conceptual design of a wide field imager onboard a 6u class cubesat platform for the study of grb prompt and afterglow emission and detection of electromagnetic counterparts of gravitational waves in soft x-rays. the planned instrument configuration consists of an array of x-ray hybrid cmos detectors (hcd), chosen for their soft-x-ray response, flexible and rapid readout rate, and low power, which makes these detectors well suited for detecting bright transients on a cubesat platform. the wide field imager is realized by a 2d coded mask. we will give an overview of the instrument design and the scientific requirements of the proposed mission | blackcat cubesat: a soft x-ray sky monitor, transient finder, and burst detector for high-energy and multimessenger astophysics |
context. deriving physical parameters from gamma-ray burst (grb) afterglow observations remains a challenge, even 20 years after the discovery of afterglows. the main reason for the lack of progress is that the peak of the synchrotron emission is in the sub-mm range, thus requiring radio observations in conjunction with x-ray/optical/near-infrared data in order to measure the corresponding spectral slopes and consequently remove the ambiguity with respect to slow vs. fast cooling and the ordering of the characteristic frequencies.aims: we have embarked on a multifrequency, multi-epoch observing campaign to obtain sufficient data for a given grb that allows us to test the simplest version of the fireball afterglow model.methods: we observed grb 151027b, the 1000th swift-detected grb, with grond in the optical-near-ir, alma in the sub-millimeter, atca in the radio band; we combined this with public swift/xrt x-ray data.results: while some observations at crucial times only return upper limits or surprising features, the fireball model is narrowly constrained by our data set, and allows us to draw a consistent picture with a fully determined parameter set. surprisingly, we find rapid, large-amplitude flux density variations in the radio band which are extreme not only for grbs, but generally for any radio source. we interpret them as scintillation effects, though their extreme nature requires the scattering screen to be at a much smaller distance than usually assumed, multiple screens, or a combination of the two.conclusions: the data are consistent with the simplest fireball scenario for a blast wave moving into a constant-density medium, and slow-cooling electrons. all fireball parameters are constrained at or better than a factor of 2, except for the density and the fraction of the energy in the magnetic field which has a factor of 10 uncertainty in both directions. this paper makes use of the following data: atca: proposal c2955 (pi: greiner), alma: ads/jao.alma#2015.1.01558.t (pi: schulze). | large-amplitude late-time radio variability in grb 151027b |
the x-ray light curves of the early afterglow phase from gamma-ray bursts (grbs) present a puzzling variability, including flares. the origin of these flares is still debated, and often associated with a late activity of the central engine. we discuss an alternative scenario where the central engine remains short-lived and flares are produced by the propagation of a long-lived reverse shock in a stratified ejecta. here we focus on the hydrodynamics of the shock interactions. we perform one-dimensional ultrarelativistic hydrodynamic simulations with different initial internal structure in the grb ejecta. we use them to extract bolometric light curves and compare with a previous study based on a simplified ballistic model. we find a good agreement between both approaches, with similar slopes and variability in the light curves, but identify several weaknesses in the ballistic model: the density is underestimated in the shocked regions, and more importantly, late shock reflections are not captured. with accurate dynamics provided by our hydrodynamic simulations, we confirm that internal shocks in the ejecta lead to the formation of dense shells. the interaction of the long-lived reverse shock with a dense shell then produces a fast and intense increase of the dissipated power. assuming that the emission is due to the synchrotron radiation from shock-accelerated electrons, and that the external forward shock is radiatively inefficient, we find that this results in a bright flare in the x-ray light curve, with arrival times, shapes, and duration in agreement with the observed properties of x-ray flares in grb afterglows. | relativistic simulations of long-lived reverse shocks in stratified ejecta: the origin of flares in grb afterglows |
the observed light curves and estimated sky rate of fast extragalactic x-ray transients (xrts) discovered in archival chandra data indicate that they belong to two distinct xrt populations. the first population of relatively short duration pulses, which typically last less than few minutes seems to be pulses of x-ray flashes, which are nearby long-duration gamma-ray bursts viewed from far off-axis. the second population of much longer pulses, which typically last hours, seems to be the early-time isotropic afterglows of short gamma-ray bursts that are beamed away from earth, as was shown in a previous paper. | fast extragalactic x-ray transients from gamma-ray bursts viewed far off-axis |
the international gamma-ray astrophysics laboratory (integral) has detected the short gamma-ray burst (sgrb), grb170817a with a signal-to-noise ratio of 4.6 and demonstrated its association with the binary neutron star merging event gw170817 detected by the ligo and virgo gravitational wave observatories. the association was immediately evident due to the timing and positional coincidence of this event with the initial error box, derived from gravitational wave (gw) measurements and the contemporaneous detection of a similar gamma-ray signal by fermi/gbm. this sgrb was detected by the integral spi acs about 1.7 s after the end of the gw emission, with a fluence of (1.4 ±0.4 ) ×10-7ergcm-2 in the 75-2000 kev energy range. after the serendipitous detection of the short prompt grb, integral continued the planned observation for about 20 h, and then performed a targeted follow-up too observation lasting several days. this too observation provided a stringent upper limit on any electromagnetic signal in a very broad energy range, from 3 kev to 8 mev, in particular constraining the soft gamma-ray afterglow flux to <7.1 ×-11ergcm-2s-1 in the range 80-300 kev. exploiting the unique capabilities of integral, we constrained the gamma-ray line emission intensity from radioactive decays expected to be the principal source of the energy behind a kilonova event following a ns-ns coalescence. finally, we put a stringent upper limit on any delayed bursting activity, for example, from a newly formed magnetar. the integral prompt detection and the subsequent continuous observations at all wavelengths have provided important constraints on the high energy emission of the resulting kilonova and the post inspiral object: ns, bh, or a new exotic object. | integral search for gw counterparts and the grb170817a/gw170817 detection |
millisecond magnetars are often invoked as the central engine of some gamma-ray bursts (grbs), specifically the ones showing a plateau phase. we argue that an apparent plateau phase may not be realized if the magnetic field of the nascent magnetar is in a transient rapid decay stage. some grbs that lack a clear plateau phase may also be hosting millisecond magnetars. we present an approximate analytical solution of the coupled set of equations describing the evolution of the angular velocity and the inclination angle between rotation and magnetic axes of a neutron star in the presence of a corotating plasma. we also show how the solution can be generalized to the case of evolving magnetic fields. we determine the evolution of the spin period, inclination angle, magnetic dipole moment, and braking index of six putative magnetars associated with grb 091018, grb 070318, grb 080430, grb 090618, grb 110715a, and grb 140206a through fitting, via bayesian analysis, the x-ray afterglow light curves by using our recent model. we find that within the first day following the formation of the millisecond magnetar, the inclination angle aligns rapidly, the magnetic dipole moment may decay by a few times, and the braking index varies by an order of magnitude. | the initial evolution of millisecond magnetars: an analytical solution |
we present a study of 21 dark gamma-ray burst (grb) host galaxies, predominantly using x-ray afterglows obtained with the chandra x-ray observatory (cxo) to precisely locate the burst in deep hubble space telescope (hst) imaging of the burst region. the host galaxies are well-detected in f160w in all but one case and in f606w imaging in 60 per cent of cases. we measure magnitudes and perform a morphological analysis of each galaxy. the asymmetry, concentration, and ellipticity of the dark burst hosts are compared against the host galaxies of optically bright grbs. in agreement with other studies, we find that dark grb hosts are redder and more luminous than the bulk of the grb host population. the distribution of projected spatial offsets for dark grbs from their host galaxy centroids is comparable to that of optically bright bursts. the dark grb hosts are physically larger, more massive and redder, but are morphologically similar to the hosts of bright grbs in terms of concentration and asymmetry. our analysis constrains the fraction of high redshift (z > 5) grbs in the sample to 14 per cent, implying an upper limit for the whole long-grb population of ≤4.4 per cent. if dust is the primary cause of afterglow darkening amongst dark grbs, the measured extinction may require a clumpy dust component in order to explain the observed offset and ellipticity distributions. | chandra and hubble space telescope observations of dark gamma-ray bursts and their host galaxies |
grb 131108a is a bright long gamma-ray burst (grb) detected by the large area telescope and the gamma-ray burst monitor on board the fermi gamma-ray space telescope. dedicated temporal and spectral analyses reveal three γ-ray flares dominating above 100 mev, which are not directly related to the prompt emission in the gamma-ray burst monitor band (10 kev-10 mev). the high-energy light curve of grb 131108a (100 mev-10 gev) shows an unusual evolution: a steep decay, followed by three flares with an underlying emission, and then a long-lasting decay phase. the detailed analysis of the γ-ray flares finds that the three flares are 6-20 times brighter than the underlying emission and are similar to each other. the fluence of each flare, (1.6 ∼ 2.0) × 10-6 erg cm-2, is comparable to that of emission during the steep decay phase, 1.7 × 10-6 erg cm-2. the total fluence from three γ-ray flares is 5.3 × 10-6 erg cm-2. the three γ-ray flares show properties similar to the usual x-ray flares that are sharp flux increases, occurring in ∼50% of afterglows, in some cases well after the prompt emission. also, the temporal and spectral indices during the early steep decay phase and the decaying phase of each flare show the consistency with a relation of the curvature effect (\hat{α } = 2 + \hat{β }), which is the first observational evidence of the high-latitude emission in the gev energy band. | bright gamma-ray flares observed in grb 131108a |
considering the contribution of emission from the host galaxies of gamma-ray bursts (grbs) to radio afterglows, we investigate the effect of host galaxies on observations statistically. for the three types of event, i.e. low-luminosity, standard and high-luminosity grbs, it is found that a tight correlation exists between the ratio of the radio flux (rrf) of the host galaxy to the total radio peak emission and the observational frequency. towards lower frequencies, in particular, the contribution from the host increases significantly. the correlation can be used to obtain a useful estimate for the radio brightness of those host galaxies that only have very limited radio afterglow data. using this prediction, we reconsidered the theoretical radio afterglow light curves for four kinds of event: high-luminosity, low-luminosity, standard and failed grbs, taking into account the contribution from host galaxies and aiming to explore the detectability of these events by the five-hundred-metre aperture spherical radio telescope (fast). lying at a typical redshift of z = 1, most of the events can be detected easily by fast. for the less fierce low-luminosity grbs, their radio afterglows are not strong enough to exceed the sensitivity limit of fast at such distances. however, since a large number of low-luminosity bursts actually happen very near to us, it is expected that fast will still be able to detect many of them. | radio afterglows and host galaxies of gamma-ray bursts |
gw170817, the first neutron star merger event detected by advanced ligo/virgo detectors, was associated with the underluminous short-duration grb 170817a. in this letter we compare the forward shock afterglow emission of gw170817/grb 170817a to other luminous short gamma-ray bursts (sgrbs) with both a known redshift and an afterglow emission lasting at least one day after the burst. in the rapid decay phase, the afterglow emission of the bright sgrbs and gw170817/grb 170817a form a natural and continuous sequence, though separated by an observation time gap. if viewed on-axis, in the bursters’ frames the forward shock afterglow emission of gw170817/grb 170817a would be among the brightest detected thus far. this provides strong evidence for the gw170817-like merger origin of bright sgrbs, and suggests that the detection of the forward shock afterglow emission of most neutron star merger events are more challenging than the case of gw170817, as usually the mergers will be more distant and the viewing angles are plausibly higher. | late afterglow emission statistics: a clear link between gw170817 and bright short gamma-ray bursts |
the joint detection of the gravitational wave signal and the electromagnetic emission from a binary neutron star merger can place unprecedented constraint on the equation of state of supranuclear matter. although a variety of electromagnetic counterparts have been observed for gw170817, including a short gamma-ray burst, kilonova, and the afterglow emission, the nature of the merger remnant is still unclear, however. the x-ray plateau is another important characteristics of short gamma-ray bursts. this plateau is probably due to the energy injection from a rapidly rotating magnetar. we investigate what we can learn from the detection of a gravitational wave along with the x-ray plateau. in principle, we can estimate the mass of the merger remnant if the x-ray plateau is caused by the central magnetar. we selected eight equations of state that all satisfy the constraint given by the gravitational wave observation, and then calculated the mass of the merger remnants of four short gamma-ray bursts with a well-measured x-ray plateau. if, on the other hand, the mass of the merger remnant can be obtained by gravitational wave information, then by comparing the masses derived by these two different methods can further constrain the equation of state. we discuss the possibility that the merger product is a quark star. in addition, we estimate the possible mass range for the recently discovered x-ray transient cdf-s xt2 that probably originated from a binary neutron star merger. finally, under the assumption that the post-merger remnant of gw170817 was a supramassive neutron star, we estimated the allowed parameter space of the supramassive neutron star and find that in this case, the magnetic dipole radiation energy is so high that it may have some effects on the short gamma-ray burst and kilonova emission. the lack of detection of these effects suggests that the merger product of gw170817 may not be a supermassive neutron star. | protomagnetar research through an analysis of the x-ray plateau in the multi-messengar era |
synchrotron emission polarization is very sensitive to the magnetic field configuration. recently, polarization of synchrotron emission with a mixed (sm) magnetic field in the gamma-ray burst (grb) afterglow phase has been developed. here, we apply these sm models to the grb prompt phase and compare their polarization properties with that of synchrotron emission in purely ordered (so) magnetic field. we find that the polarization properties in an sm model are very similar to these in a corresponding so model (e.g., synchrotron emission in a mixed magnetic field with an aligned ordered part (sma) and synchrotron emission with a purely ordered aligned magnetic field (soa)), only with a lower polarization degree (pd). we also discuss the statistical properties of the models. we find pds of the simulated bursts are concentrated around 25% for both soa and synchrotron emission in a purely ordered toroidal magnetic field (sot), while they can range from 0% to 25% for sma and synchrotron emission in a mixed magnetic field with a toroidal ordered part (smt), depending on ξb value, i.e., the ratio of magnetic reduction of the ordered magnetic field over that of random magnetic field. from statistics, if pds of majority grbs are non-zero, then it favors so and sm models. further, if there are some bright grbs with prominently lower pds than that of the majority grbs, it favors sot (smt) models; if all the bright grbs have comparable pds with the majority ones, it favors soa (sma) models. finally, we apply our results to polar's data and find that ∼ 10% time-integrated pds of the observed bursts favor sma and smt models, and the ξb parameter of these bursts is constrained to be around 1.135. | polarization of grb prompt emission and its application to polar's data |
electromagnetic observations of the first binary neutron star (bns) merger detected in gravitational waves, gw170817, have established that relativistic jets can be successfully launched in bns mergers. typically, such jets produce emission in two phases: γ-ray prompt emission and multiwavelength afterglow. because of relativistic beaming and the jet's angular structure, the detectability of both these counterparts is dependent on the angle (θ${\rm v}$) between the observer's line of sight and the jet axis. we compare the detectability of prompt and afterglow emission from off-axis jets, assuming standard detector thresholds such as that of fermi gamma-ray burst monitor (gbm), chandra, and jansky very large array (vla). we find that for top-hat jets, afterglow is a more likely counterpart than the prompt emission even with unfavourable afterglow parameters. for structured jets with a gaussian profile, prompt emission is more promising than the afterglows at extreme viewing angles, under the assumption that the total energy emitted in the prompt phase equals the kinetic energy of the outflow. assuming a gaussian jet profile, we forecast the population of γ-ray detections and find that extreme viewing angle events like grb 170817a will be rare. in our simulated sample, the observed isotropic equivalent energy in γ-rays is moderately correlated with the viewing angle, such that a low eiso,γ is almost always associated with a high off-axis viewing angle. | detectability of electromagnetic counterparts from neutron star mergers: prompt emission versus afterglow |
we present a spectroscopic investigation with the very large telescope/x-shooter of seven candidate extremely strong damped lyman-α absorption systems [esdlas, n(h i) ≥ 5 × 1021 cm-2] observed along quasar sightlines. we confirm the extremely high column densities, albeit slightly (0.1 dex) lower than the original esdla definition for four systems. we measured low-ionization metal abundances and dust extinction for all systems. for two systems, we also found strong associated h$\rm _2$ absorption $\log n(\rm{h\rm _2})\,\text{(cm$^{-2}$)}=18.16\pm 0.03$ and 19.28 ± 0.06 at z = 3.26 and 2.25 towards j2205+1021 and j2359+1354, respectively, while for the remaining five we measured conservative upper limits on the h$\rm _2$ column densities of typically $\log n(\rm{h\rm _2})\,\text{(cm$^{-2}$)}\,\lt\, 17.3$. the increased h2 detection rate (10-55 per cent at 68 per cent confidence level) at high h i column density compared with the overall damped lyman-α population (~5-10 per cent) confirms previous works. we find that these seven esdlas have similar observed properties as those previously studied towards quasars and γ-ray burst afterglows, suggesting they probe inner regions of galaxies. we use the abundance of ionized carbon at the excited fine-structure level to calculate the cooling rates through the c ii λ158 μm emission, and compare them with the cooling rates from damped lyman-α systems in the literature. we find that the cooling rate distribution of esdlas also presents the same bimodality as previously observed for the general (mostly lower h i column density) damped lyman-α population. | extremely strong dlas at high redshift: gas cooling and h2 formation |
long gamma-ray bursts (grbs) are considered to originate from core collapse of massive stars. it is believed that the afterglow property is determined by the density of the material in the surrounding interstellar medium (ism). therefore, the circumburst density can be used to distinguish between an interstellar wind, n(r) ∝ r -k , and a constant-density medium (ism), $n(r)=\mathrm{const}$ . previous studies with different afterglow samples show that the circumburst medium of grbs is neither simply supported by an interstellar wind nor completely favored by an ism. in this work, our new sample consists of 39 grbs with smoothly onset bump-like features in early x-ray afterglows, in which 20 grbs have the redshift measurements. by using a smooth broken power-law function to fit the bumps of x-ray light curves, we derive the fwhm as the feature width (ω), as well as the rise and decay timescales of the bumps (trand td ). the correlations between the timescales of x-ray bumps are similar to those found previously in the optical afterglows. based on the fireball forward shock model of the thin shell case, we obtain the distribution of the electron spectral index p and further constrain the medium density distribution index k. the new inferred k is found to be concentrated at 1.0, with a range from 0.2 to 1.8. this finding is consistent with previous studies. the conclusion of our detailed investigation for x-ray afterglows suggests that the ambient medium of the selected grbs is not homogeneous, i.e., neither ism nor the typical interstellar wind. | constraining the circumburst medium of gamma-ray bursts with x-ray afterglows |
studies of high-redshift gamma-ray bursts (grbs) provide important information about the early universe such as the rates of stellar collapsars and mergers, the metallicity content, constraints on the re-ionization period, and probes of the hubble expansion. rapid selection of high-z candidates from grb samples reported in real time by dedicated space missions such as swift is the key to identifying the most distant bursts before the optical afterglow becomes too dim to warrant a good spectrum. here, we introduce `machine-z', a redshift prediction algorithm and a `high-z' classifier for swift grbs based on machine learning. our method relies exclusively on canonical data commonly available within the first few hours after the grb trigger. using a sample of 284 bursts with measured redshifts, we trained a randomized ensemble of decision trees (random forest) to perform both regression and classification. cross-validated performance studies show that the correlation coefficient between machine-z predictions and the true redshift is nearly 0.6. at the same time, our high-z classifier can achieve 80 per cent recall of true high-redshift bursts, while incurring a false positive rate of 20 per cent. with 40 per cent false positive rate the classifier can achieve ∼100 per cent recall. the most reliable selection of high-redshift grbs is obtained by combining predictions from both the high-z classifier and the machine-z regressor. | machine-z: rapid machine-learned redshift indicator for swift gamma-ray bursts |
particle acceleration is expected to be different between relativistic and non-relativistic collisionless shocks. we show that electromagnetic counterparts to gravitational waves (gws), gamma-ray burst (grb) afterglows, are ideal targets for observing trans-relativistic evolution of accelerated electron distribution because the gws spot nearby grbs with off-axis jets, otherwise missed in gamma-ray observations. we find that the relativistic spectral slope begins to change steeply near the peak time of the light curve and approaches the non-relativistic limit in about 10 times the peak time. the trans-relativistic evolution of the afterglow synchrotron spectrum is consistent with grb 170817a observations within errors, and will be measurable in similar but more distant events at a gw horizon ~200 mpc in a denser environment. we roughly estimate that such events represent a fraction of 10-50 per cent of the grb 170817a-like off-axis short grbs. we also find that the spectral evolution does not depend on the jet structure if their light curves are similar to each other. | probing particle acceleration at trans-relativistic shocks with off-axis gamma-ray burst afterglows |
grb 130925a is an ultra-long gamma-ray burst (grb), and it shows clear evidence for thermal emission in the soft x-ray data of the swift/x-ray telescope (xrt; ∼0.5 kev), lasting until the x-ray afterglow phase. due to the long duration of the grb, the burst could be studied in hard x-rays with high-resolution focusing detectors (nustar). the blackbody temperature, as measured by the swift/xrt, shows a decreasing trend until the late phase (piro et al.) whereas the high-energy data reveal a significant blackbody component during the late epochs at an order of magnitude higher temperature (∼5 kev) compared to contemporaneous low energy data (bellm et al.). we resolve this apparent contradiction by demonstrating that a model with two black bodies and a power law (2bbpl) is consistent with the data right from the late prompt emission to the afterglow phase. both blackbodies show a similar cooling behavior up to late times. we invoke a structured jet, having a fast spine and a slower sheath layer, to identify the location of these blackbodies. independent of the physical interpretation, we propose that the 2bbpl model is a generic feature of the prompt emission of all long grbs, and the thermal emission found in the afterglow phase of different grbs reflects the lingering thermal component of the prompt emission with different timescales. we strengthen this proposal by pointing out a close similarity between the spectral evolutions of this grb and grb 090618, a source with significant wide band data during the early afterglow phase. | thermal emissions spanning the prompt and the afterglow phases of the ultra-long grb 130925a |
we present a sample composed of the 41 faintest x-ray afterglows of the population of long gamma-ray bursts (lgrbs) with known redshift. we study their intrinsic properties (spectral index, decay index, distance, luminosity, isotropic radiated energy, and peak energy) and their luminosity distribution functions to assess whether they belong to the same population as the brighter afterglow events. we find that these events belong to a population of nearby ones, different from the general population of lgrbs. in addition, these events are faint during their prompt phase, and include the few possible outliers of the amati relation. | a study of grbs with low-luminosity afterglows |
in this paper, we revisit the scenario that an internal gradual magnetic dissipation taking place within the wind from a newborn millisecond magnetar can be responsible for gamma-ray burst (grb) production. we show that a combination of two emission components in this model, i.e., the photospheric emission from the wind and the synchrotron radiation within the magnetic reconnection region, can give a reasonable fit to the observed spectrum of the prompt emission phase of grb 160804a. we obtain the physical parameters through a monte carlo procedure and deduce the initial spin period and magnetic field of the central magnetar. furthermore, the independent afterglow fitting analysis gives a consistent result, adding great credibility to this scenario. in addition, we predict a subclass of grbs from such magnetar wind internal gradual magnetic dissipation (abbreviated as “migmad bursts”) that have several distinctive properties. | prompt emission of gamma-ray bursts from the wind of newborn millisecond magnetars: a case study of grb 160804a |
using multiwavelength observations of radio afterglows, we confirm the hypothesis that the flux density of gamma-ray bursts (grbs) at a fixed observing frequency is invariable when the distance of the grbs increases, which means the detection rate will be approximately independent of redshift. we study this behavior theoretically and find that it can be well explained by the standard forward shock model involving a thin shell expanding in either a homogeneous interstellar medium (ism) or a wind environment. we also found that short grbs and supernova-associated grbs, which are at relatively smaller distances, marginally match the flux-redshift relationship and they could be outliers. we rule out the assumption that the medium density evolves with redshift as n ∝ (1 + z)4 from the current measurements of n and z for short and long grbs. in addition, the possible dependence of host flux on the redshift is also investigated. we find that a similar redshift independence of the flux exists for host galaxies as well, which implies that the detection rate of radio hosts might also be independent of the redshift. it is also hinted that most radio hosts have spectral indices ranging from βh≃ -1 to 2.5 in statistics. finally, we predict the detection rates of radio afterglows by next-generation radio telescopes such as the five-hundred meter aperture spherical telescope (fast) and the square kilometer array (ska). | the redshift dependence of the radio flux of gamma-ray bursts and their host galaxies |
a generic synchrotron external shock model is the widely preferred paradigm used to interpret the broad-band afterglow data of gamma-ray bursts (grbs), including predicted observable signatures from a reverse shock which have been confirmed by observations. investigations of the nature of the reverse shock emission can provide valuable insights into the intrinsic properties of the grb ejecta. here we briefly review the standard and the extended models of the reverse shock emission, discussing the connection between the theory and observations, including the implications of the latest observational advances. | reverse shock emission in gamma-ray bursts revisited |
from a comprehensive statistical analysis of swift x-ray light curves of gamma-ray bursts (grbs) collected from 2004 december to the end of 2010, we found a three-parameter correlation between the isotropic energy emitted in the rest-frame 1-104 kev energy band during the prompt emission (eγ,iso), the rest-frame peak of the prompt emission energy spectrum (epk), and the x-ray energy emitted in the rest-frame 0.3-30 kev observed energy band (ex,iso), computed excluding the contribution of the flares. in this paper, we update this correlation with the data collected until 2014 june, expanding the sample size with ∼35 per cent more objects, where the number of short grbs doubled. with this larger sample, we confirm the existence of a universal correlation that connects the prompt and afterglow properties of long and short grbs. we show that this correlation does not depend on the x-ray light-curve morphology and that further analysis is necessary to firmly exclude possible biases derived by redshift measurements. in addition, we discuss about the behaviour of the peculiar objects as ultra-long grbs and we propose the existence of an intermediate group between long and short grbs. interestingly, two grbs with uncertain classification fall into this category. finally, we discuss the physics underlying this correlation, in the contest of the efficiency of conversion of the prompt gamma-ray emission energy into the kinetic energy of the afterglow, the photospheric model, and the cannonball model. | update on the grb universal scaling ex,iso-eγ,iso-epk with 10 years of swift data |
we present a brief history and the current status of research on gamma-ray bursts (grbs), including observations at various grb phases: the prompt emission, afterglow, supernova/kilonova, and the host galaxy. an overview of the known phenomenological features and theoretical models for grbs is given. problems, the solution of which is necessary for a successful study of the phenomenon of grb, are discussed. | gamma-ray bursts: multiwavelength investigations and models |
we present 5.5 ghz observations with the very large array of a sample of nearby galaxies with energetic nuclear outbursts at mid-infrared (mir) bands. these observations reach a uniform depth down to a median rms of ∼10 μjy, representing one of the most sensitive searches for radio emission associated with nuclear transients. we detect radio emission in 12 out of 16 galaxies at a level of >5σ, corresponding to a detection rate of 75%. such a high detection is remarkably different from previous similar searches in stellar tidal disruption events. the radio emission is compact and not resolved for the majority of sources on scales of ≲0"5 (<0.9 kpc at z < 0.1). we find that the possibility of the star formation contributing to the radio emission is low, but an active galactic nucleus (agn) origin remains a plausible scenario, especially for sources that show evidence of agn activity in their optical spectra. if the detections could represent radio emission associated with a nuclear transient phenomenon such as a jet or outflow, we could use the blast wave model by analogy with the gamma-ray burst afterglows to describe the evolution of radio light curves. in this context, the observations are consistent with a decelerating jet with an energy of ∼1051-52 erg viewed at 30°-60° off-axis at later times, suggesting that powerful jets may be ubiquitous among mir-burst galaxies. future continuous monitoring observations will be crucial to decipher the origin of radio emission through detections of potential flux and spectral evolution. our results highlight the importance of radio observations to constrain the nature of nuclear mir outbursts in galaxies. | compact radio emission from nearby galaxies with mid-infrared nuclear outbursts |
the gamma-ray burst (grb) afterglows provide a unique opportunity to study the interstellar medium (ism) of star-forming galaxies at high-redshift. the grb-dlas (damped lyman-α absorbers) contain a large neutral hydrogen column density, n(h i), and are observed against the grb afterglow. a large fraction of grb-dlas show presence of molecular hydrogen (h2) which is an indicator of star-formation. hence it is important to study those grb-dlas which have h2 lines to decipher and understand their physical conditions. the grb-dlas 121024a and 120815a, situated at redshift 2.30 and 2.36, respectively, are two such important h2-bearing grb-dlas. besides h2, these two grb-dlas also show many metal lines. in this work we have carried out a detail numerical study on the h2 lines, as well as on those metal lines, in grb-dlas 121024a and 120815a self-consistently. we use the spectral synthesis code cloudy for this study. this modelling helps us to determine the underlying physical conditions which give rise to such lines and hence to understand these two grb-dlas in much more detail than any other previous investigation. we find that the hydrogen densities for these two h2-bearing dlas are ≥60 cm-3. moreover our study infers that the linear sizes are ≤17.7 pc for these two grb-dlas, and the mean gas temperatures averaged over the cloud thickness, are ≤140 k. overall, we find that these two h2-bearing grb-dlas are denser, cooler, and smaller compared to those without h2. | physical conditions in two high-redshift h2-bearing grb-dlas, 120815a and 121024a |
we investigate the multi-band properties of the grb 161017a with z = 2.013, which was detected by swift and fermi satellites, and other instruments. the optical and x-ray afterglows were all detected at early times after the prompt emission. the optical light curve shows a very bright onset peak at about 100 s for 13 mag of r-band, while the x-ray light curve occurs several flares at the beginning. we investigate the origin of x-ray and optical afterglows by analyzing the optical and x-ray data. considering the smooth onset bump in the early time of the optical band and the erratic pulses for the x-ray lightcurve, we suppose that the early optical afterglow may be produced by the external shock, while the early time of x-ray light curve is dominated by flares. therefore, grb optical afterglows with smooth onset bump features at early time are possibly produced by external — forward shock (fs). according to the fireball external-model, the temporal slopes of the onset bumps are determined by both the medium density profile and the electron spectral index. therefore, the afterglow onset bumps would be an ideal probe to study the properties of the fireball and the circumburst medium. the density profile has a slope of k = 0.74 ± 0.08, which suggests that the circumburst environment of the grb 161017a would be an intermediate regime that are between the homogeneous interstellar medium (ism) and wind-type medium. in addition, the optical light curve of the grb 161017a exhibits a plateau feature and rebrightening after the onset bump, which may be related to the long-acting central engine of grbs. | grb 161017a, the circumburst environment is an intermediate regime between the homogeneous interstellar medium and wind-type medium |
observations of short-duration gamma-ray bursts and their afterglows show that a good fraction (perhaps ≳50%) of binary neutron star mergers lead to strongly magnetized, rapidly rotating pulsars (including millisecond magnetars), no matter whether the pulsar remnants are short- or long-lived. such compact objects are very likely to have significant radial oscillations and high interior temperatures, as indicated in recent numerical simulations. in this paper, we have investigated rotation-induced gravitational radiation from possibly existing, radially oscillating pulsars after binary neutron star mergers, and find that this mechanism can efficiently damp the radial oscillations. the resulting gravitational waves (gws) could have a non-negligible contribution to the high-frequency spectrum. we provide an order-of-magnitude estimate of the event rate and suggest that such gw events would be detectable with the advanced ligo/virgo or next-generation detectors. our discussion can also be applied to newborn, radially oscillating, millisecond pulsars formed through the other astrophysical processes. | gravitational waves from post-merger radially oscillating millisecond pulsars |
we use published data in radio, optical, and x-ray bands to analyse and model afterglows of gw/grb 170817a. our analysis is based on a phenomenological gamma-ray burst generator model, which we previously used to study the prompt gamma-ray emission of this important transient. we find a multicomponent model and a few of its variants that are consistent with broad-band ∼1 yr observations of afterglows, once the contribution of kilonova in optical/ir band is taken into account. considering beaming and off-axis view of relativistic outflows, we interpret the components of the model as approximately presenting the profile of a relativistic structured jet with a rapidly declining lorentz factor from our line of sight, where it had a lorentz factor of o(100), to outer boundaries, where it became a mildly relativistic cocoon with a relative velocity to light of ∼0.4-0.97. properties of the ultra-relativistic core of the jet obtained here are consistent with conclusions from analysis of the prompt gamma-ray emission. in particular, our results show that after prompt internal shocks the remnant of the jet retained in some extent its internal collimation and coherence. slow rise of the afterglows can be associated to low density of circumburst material and low column density of the jet. the long distance of external shocks from the merger, which could have been in part responsible for extensive thinning of the jet through expansion and energy dissipation before occurrence of external shocks, is responsible for the peak of emission being at ≳110 d after the merger. we discuss implications of these observations for origin and properties of circumburst material around binary neutron stars. this analysis confirms our previous results showing that an outflow with a lorentz factor of ∼2-5 cannot explain observed afterglows without an additional x-ray source or significant absorption of optical/ir photons. | properties of jet and surrounding material of gw/grb 170817a |
there exists an inevitable scatter in intrinsic luminosity of gamma ray bursts(grbs). if there is relativistic beaming in the source, viewing angle variation necessarily introduces variation in the intrinsic luminosity function(ilf). scatter in the ilf can cause a selection bias where distant sources that are detected have a larger median luminosity than those detected close by. median luminosity, as we know, divides any given population into equal halves. when the functional form of a distribution is unknown, it can be a more robust diagnostic than any that use trial functional forms. in this work we employ a statistical test based on median luminosity and apply it to test a class of models for grbs. we assume that the grb jet has a finite opening angle and that the orientation of the grb jet is random relative to the observer. we parameterize the jet with constant lorentz factor $\gamma$ and opening angle $\theta_0$. we calculate $l_{median}$ as a function of redshift with an average of 17 grbs in each redshift bin($dz=0.01$) empirically, theoretically and use fermi gbm data, noting that swift data is problematic as it is biased, specially at high redshifts. we find that $l_{median}$ is close to $l_{max}$ for sufficiently extended grb jet and does not fit the data. we find an acceptable fit with the data when $\gamma$ is between $100$ and $200$, $\theta_0\leq 0.1$, provided that the jet material along the line of sight to the on axis observer is optically thick, such that the shielded maximum luminosity is well below the bare $l_{max}$. if we associate an on-axis observer with a classically projected monotonically decreasing afterglow, we find that their ilf is similar to those of off-jet observer which we associate with flat phase afterglows. | luminosity selection for gamma ray bursts |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.