Short-hard Gamma-ray Bursts Research Articles

Alternative statistical interpretation for the apparent plateaus in the duration distributions of gamma-ray bursts

Context. The existence of a plateau in the short-duration tail of the observed distribution of cosmological long-soft gamma ray bursts (LGRBs) is posited to be the first direct evidence of collapsars. A similar plateau in the short-duration tail of the observed duration distribution of short-hard gamma ray bursts (SGRBs) has been suggested as evidence of compact binary mergers. Aims. We present an equally plausible alternative interpretation for this evidence, based on a purely statistical approach. Methods. Specifically, we show that the observed plateau in the short-duration tail of the duration distribution of LGRBs can naturally occur in the statistical distributions of strictly positive physical quantities, exacerbated by the effects of mixing with the duration distribution of SGRBs, observational selection effects, and data aggregation (e.g., binning) methodologies. The observed plateau in the short-duration tail of the observed distributions of SGRBs may likewise result from a combination of sample incompleteness and inhomogeneous binning of data. We further confirm the impact of these factors on the observation of a plateau in the duration distributions of GRBs through extensive numerical Monte Carlo simulations. Results. This analysis corroborates and strengthens a purely statistical and sample-incompleteness interpretation of the observed plateau in the duration distribution of LGRBs and SGRBs, without invoking the physics of collapsars or jet-propagation through the stellar envelope.

Magnetar Giant Flare Origin for GRB 210410A?

In general, giant flares (GFs) produced by magnetars have a very short-hard initial spike that is followed by a weak oscillatory phase. GFs from a nearby galaxy would appear as cosmic short-hard gamma-ray bursts (GRBs), such as GRB 200415A. In this paper, we search for GF-originated bursts in the Fermi GRB category and report GRB 210410A, which is presented with a very short-hard spike followed by an extended tail emission. In the E p,z − E iso plane, GRB 210410A with a duration of T 90 ∼ 48 s differs from long GRBs, might be classified as a short GRB with a redshift of z ∼ 0.28, and could be regarded as a GF with a distance of ∼2.7 Mpc. Here, E p,z , E iso, and L iso denote the rest-frame peak photon energy, the isotropic energy, and the isotropic luminosity of the burst, respectively. The radiation spectrum of GRB 210410A, similar to that of GRB 200415A, can be well fitted with a non-dissipative photospheric emission. However, GRB 210410A in the E p,z − L iso plane is beyond the death line of cosmic GRBs for non-dissipated photospheric emission with a general initial size of the fireball. Since the E p,z − L iso relation of GFs is far beyond the death line of cosmic GRBs, GRB 210410A may have originated from the same channel that produces GFs. We also perform the analysis and discuss both the highest photon energy event (4.2 GeV) and the extended tail emission in this burst.

General-relativistic neutrino-radiation magnetohydrodynamic simulation of seconds-long black hole-neutron star mergers

Seconds-long numerical-relativity simulations for black hole-neutron star mergers are performed for the first time to obtain a self-consistent picture of the merger and post-merger evolution processes. To investigate the case that tidal disruption takes place, we choose the initial mass of the black hole to be $5.4M_\odot$ or $8.1M_\odot$ with the dimensionless spin of 0.75. The neutron-star mass is fixed to be $1.35M_\odot$. We find that after the tidal disruption, dynamical mass ejection takes place spending $\lesssim 10$ ms together with the formation of a massive accretion disk. Subsequently, the magnetic field in the disk is amplified by the magnetic winding and magnetorotational instability, establishing a turbulent state and inducing the angular momentum transport. The post-merger mass ejection by the magnetically-induced viscous effect sets in at $\sim 300$-500\,ms after the tidal disruption, at which the neutrino luminosity drops below $\sim 10^{51.5}\,{\rm erg/s}$, and continues for several hundreds ms. A magnetosphere near the rotational axis of the black hole is developed after the matter and magnetic flux fall into the black hole from the accretion disk, and high-intensity Poynting flux generation sets in at a few hundreds ms after the tidal disruption. The intensity of the Poynting flux becomes low after the significant post-merger mass ejection, because the opening angle of the magnetosphere increases. The lifetime for the stage with the strong Poynting flux is $1$-2 s, which agrees with the typical duration of short-hard gamma-ray bursts.

Critical Tests of Leading Gamma Ray Burst Theories

It has been observationally established that supernovae (SNe) of Type Ic produce long duration gamma-ray bursts (GRBs) and that neutron star mergers generate short hard GRBs. SN-Less GRBs presumably originate in a phase transition of a neutron star in a high mass X-ray binary. How these phenomena actually generate GRBs is debated. The fireball and cannonball models of GRBs and their afterglows have been widely confronted with the huge observational data, with their defenders claiming success. The claims, however, may reflect multiple choices and the use of many adjustable parameters, rather than the validity of the models. Only a confrontation of key falsifiable predictions of the models with solid observational data can test their validity. Such critical tests are reviewed in this report.

Discovery and confirmation of the shortest gamma-ray burst from a collapsar

Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the universe. The duration and hardness distribution of GRBs has two clusters, now understood to reflect (at least) two different progenitors. Short-hard GRBs (SGRBs; T90 <2 s) arise from compact binary mergers, while long-soft GRBs (LGRBs; T90 >2 s) have been attributed to the collapse of peculiar massive stars (collapsars). The discovery of SN 1998bw/GRB 980425 marked the first association of a LGRB with a collapsar and AT 2017gfo/GRB 170817A/GW170817 marked the first association of a SGRB with a binary neutron star merger, producing also gravitational wave (GW). Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi Satellite and the InterPlanetary Network (IPN) localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova (KN), but is consistent with being the supernova (SN). Despite the GRB duration being short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirms a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.

Are there radio-loud and radio-quiet gamma-ray bursts?

ABSTRACT The potential existence of two separate classes of Long-duration Gamma-Ray Bursts (LGRBs) with and without radio afterglow emission, corresponding to radio-bright/loud and radio-dark/quiet populations, has been recently argued and favoured in the GRB literature. The radio-quiet LGRBs have been found to have, on average, lower total isotropic gamma-ray emissions (Eiso) and shorter intrinsic prompt gamma-ray durations (e.g. T90z). In addition, a redshift −T90z anticorrelation has been discovered among the radio-loud LGRBs, which is reportedly missing in the radio-quiet class. Here, we discuss the significance of the differences between the energetics and temporal properties of the two proposed classes of radio-loud and radio-quiet LGRBs. We show that much of the proposed evidence in support of the two distinct radio populations of LGRBs can be explained away in terms of selection effects and sample incompleteness. Our arguments are based on the recent discovery of the relatively strong, highly significant positive correlation between the total isotropic emission (Eiso) and the intrinsic prompt duration (T90z) that is present in both populations of short-hard and long-soft GRBs, predicted, quantified, and reported for the first time by Shahmoradi (2013a;2013b) and Shahmoradi &amp; Nemiroff (2015).

Magnetar Giant Flare Origin for GRB 200415A Inferred from a New Scaling Relation

Abstract Soft gamma-ray repeaters (SGRs) are a mainly Galactic population and originate from neutron stars with intense (B ≃ 1015 G) magnetic fields (magnetars). Occasionally, a giant flare occurs with enormous intensity, displaying a short, hard spike followed by a weaker, oscillatory phase that exhibits the rotational period of the neutron star. If the magnetar giant flares occur in nearby galaxies, they would appear as cosmic, short-hard gamma-ray bursts (GRBs) without detecting the weak oscillatory phase. Recently, a short-hard GRB named GRB 200415A was detected, with a position coincident with the Sculptor Galaxy (NGC 253), raising the question of whether it is a classic short GRB or a magnetar giant flare. Here we show that magnetar giant flares follow a scaling relation between the spectral peak energy and the isotropic energy in 1 keV–10 MeV, i.e., , and locate in a distinct region of the E p–E iso plane from that of classic short GRBs. The relation can be well understood in the model that giant flares arise from the photosphere emissions of relativistically expanding fireballs. GRB 200415A, together with two other candidate giant flares (GRB 051103 and GRB 070201) follow this relation, which strongly favors the giant flare origin of these GRBs. The GeV emission detected by Fermi/LAT from GRB 200415A at 18–285 s can also be explained in the giant flare scenario. The total energy in the GeV emission implies a baryon load of ∼1023 g in the giant flare fireball of GRB 200415A.

Mass ejection from neutron-star mergers

AbstractMerger of binary neutron stars and black hole-neutron star binaries is the promising source of short-hard gamma-ray bursts, the most promising site for the r-process nucleosynthesis, and the source of kilonovae. To theoretically predict the merger and mass ejection processes and resulting electromagnetic emission, numerical simulation in full general relativity (numerical relativity) is the unique approach. We summarize our current understanding for the processes of neutron-star mergers and subsequent mass ejection based on the results of long-term numerical-relativity simulations. We pay particular attention to the electron fraction of the ejecta.

A Strong Jet Signature in the Late-time Light Curve of GW170817

Abstract We present new 0.6–10 GHz observations of the binary neutron star merger GW170817 covering the period up to 300 days post-merger, taken with the upgraded Karl G. Jansky Very Large Array, the Australia Telescope Compact Array, the Giant Metrewave Radio Telescope and the MeerKAT telescope. We use these data to precisely characterize the decay phase of the late-time radio light curve. We find that the temporal decay is consistent with a power-law slope of t −2.2, and that the transition between the power-law rise and decay is relatively sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated) outflow, but is instead the classic signature of a relativistic jet. This provides strong observational evidence that GW170817 produced a successful jet, and directly demonstrates the link between binary neutron star mergers and short-hard gamma-ray bursts. Using simple analytical arguments, we derive constraints on the geometry and the jet opening angle of GW170817. These results are consistent with those from our companion very long baseline interferometry paper, reporting superluminal motion in GW170817.

Off-axis emission of short GRB jets from double neutron star mergers and GRB 170817A

The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9$\;$days) and radio (16.4$\;$days) emission, together with the low isotropic equivalent $\gamma$-ray energy output ($E_{\rm\gamma,iso}\approx5\times10^{46}\;$erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. Here we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle $\theta_{\rm obs}\approx16^\circ-26^\circ$, GRB jet energy $E\sim10^{48.5}-10^{49.5}~{\rm erg}$, and external density $n\sim10^{-5}-10^{-1}~{\rm cm}^{-3}$ for a $\xi_e\sim 0.1$ non-thermal electron acceleration efficiency.

A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817.

GW170817 was the first gravitational-wave detection of a binary neutron-star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger (and successfully breaking out of the surrounding material), directed away from our line of sight (off-axis). The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard-γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although we cannot completely rule out the existence of a jet directed away from the line of sight, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission (and possibly also the ultraviolet and optical emission), it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.

Infrared Emission from Kilonovae: The Case of the Nearby Short Hard Burst GRB 160821B

Abstract We present constraints on Ks-band emission from one of the nearest short hard gamma-ray bursts, GRB 160821B, at z = 0.16, at three epochs. We detect a red relativistic afterglow from the jetted emission in the first epoch but do not detect any excess kilonova emission in the second two epochs. We compare upper limits obtained with Keck I/MOSFIRE to multi-dimensional radiative transfer models of kilonovae, that employ composition-dependent nuclear heating and LTE opacities of heavy elements. We discuss eight models that combine toroidal dynamical ejecta and two types of wind and one model with dynamical ejecta only. We also discuss simple, empirical scaling laws of predicted emission as a function of ejecta mass and ejecta velocity. Our limits for GRB 160821B constrain the ejecta mass to be lower than 0.03 M ⊙ for velocities greater than 0.1 c. At the distance sensitivity range of advanced LIGO, similar ground-based observations would be sufficiently sensitive to the full range of predicted model emission including models with only dynamical ejecta. The color evolution of these models shows that I–K color spans 7–16 mag, which suggests that even relatively shallow infrared searches for kilonovae could be as constraining as optical searches.

The status of general relativistic simulations of compact binary mergers as engines of short gamma-ray bursts

Black hole - neutron star (BHNS) and neutron star - neutron star (NSNS) binaries are perhaps the most popular progenitors for short-hard gamma ray bursts. After about two decades of numerical relativity simulations of binary compact objects we are beginning to understand the necessary ingredients for jets to emerge from these systems following merger. We report on the latest development of this field summarizing the results from state-of-the-art numerical relativity (magnetohydrodynamic) simulations of compact binary mergers as progenitors of short-hard gamma-ray bursts.

General relativistic simulations of compact binary mergers as engines for short gamma-ray bursts

Black hole—neutron star (BHNS) and neutron star—neutron star (NSNS) binaries are among the favored candidates for the progenitors of the black hole—disk systems that may be the engines powering short-hard gamma ray bursts. After almost two decades of simulations of binary NSNSs and BHNSs in full general relativity we are now beginning to understand the ingredients that may be necessary for these systems to launch incipient jets. Here, we review our current understanding, and summarize the surprises and lessons learned from state-of-the-art (magnetohydrodynamic) simulations in full general relativity of BHNS and NSNS mergers as jet engines for short-hard gamma-ray bursts. We also propose a new approach to probing the nuclear equation of state by virtue of multimessenger observations.

Two New Calcium-rich Gap Transients in Group and Cluster Environments

Abstract We present the Palomar Transient Factory discoveries and the photometric and spectroscopic observations of PTF11kmb and PTF12bho. We show that both transients have properties consistent with the class of calcium-rich gap transients, specifically lower peak luminosities and rapid evolution compared to ordinary supernovae, and a nebular spectrum dominated by [Ca ii] emission. A striking feature of both transients is their host environments: PTF12bho is an intracluster transient in the Coma Cluster, while PTF11kmb is located in a loose galaxy group, at a physical offset ∼150 kpc from the most likely host galaxy. Deep Subaru imaging of PTF12bho rules out an underlying host system to a limit of , while Hubble Space Telescope imaging of PTF11kmb reveals a marginal counterpart that, if real, could be either a background galaxy or a globular cluster. We show that the offset distribution of Ca-rich gap transients is significantly more extreme than that seen for SNe Ia or even short-hard gamma-ray bursts (sGRBs). Thus, if the offsets are caused by a kick, they require higher kick velocities and/or longer merger times than sGRBs. We also show that almost all Ca-rich transients found to date are in group and cluster environments with elliptical host galaxies, indicating a very old progenitor population; the remote locations could partially be explained by these environments having the largest fraction of stars in the intragroup/intracluster light following galaxy–galaxy interactions.

ASTROSAT CZT IMAGER OBSERVATIONS OF GRB 151006A: TIMING, SPECTROSCOPY, AND POLARIZATION STUDY

ABSTRACT AstroSat is a multi-wavelength satellite launched on 2015 September 28. The CZT Imager of AstroSat on its very first day of operation detected a long duration gamma-ray burst (GRB), namely GRB 151006A. Using the off-axis imaging and spectral response of the instrument, we demonstrate that the CZT Imager can localize this GRB correctly to about a few degrees, and it can provide, in conjunction with Swift, spectral parameters similar to those obtained from Fermi/GBM. Hence, the CZT Imager would be a useful addition to the currently operating GRB instruments (Swift and Fermi). Specifically, we argue that the CZT Imager will be most useful for the short hard GRBs by providing localization for those detected by Fermi and spectral information for those detected only by Swift. We also provide preliminary results on a new exciting capability of this instrument: the CZT Imager is able to identify Compton scattered events thereby providing polarization information for bright GRBs. GRB 151006A, in spite of being relatively faint, shows hints of a polarization signal at 100–300 keV (though at a low significance level). We point out that the CZT Imager should provide significant time resolved polarization measurements for GRBs that have fluence three times higher than that of GRB 151006A. We estimate that the number of such bright GRBs detectable by the CZT Imager is five to six per year. The CZT Imager can also act as a good hard X-ray monitoring device for possible electromagnetic counterparts of gravitational wave events.

Short versus long gamma-ray bursts: a comprehensive study of energetics and prompt gamma-ray correlations

We present the results of a comprehensive study of the luminosity function, energetics, prompt gamma-ray correlations, and classification methodology of short-hard and long-soft GRBs (SGRBs and LGRBs), based on observational data in the largest catalog of GRBs available to this date: BATSE catalog of 2702 GRBs. We find that: 1. The least-biased classification method of GRBs into short and long, solely based on prompt-emission properties, appears to be the ratio of the observed spectral peak energy to the observed duration ($R=E_p/T_{90}$) with the dividing line at $R\simeq50[keV~s^{-1}]$. 2. Once data is carefully corrected for the effects of the detection threshold of gamma-ray instruments, the population distribution of SGRBs and LGRBs can be individually well described as multivariate log-normal distribution in the $4$--dimensional space of the isotropic peak gamma-ray luminosity, total isotropic gamma-ray emission, the intrinsic spectral peak energy, and the intrinsic duration. 3. Relatively large fractions of SGRBs and LGRBs with moderate-to-low spectral peak energies have been missed by BATSE detectors. 4. Relatively strong and highly significant intrinsic hardness--brightness and duration--brightness correlations likely exist in both populations of SGRBs and LGRBs, once data is corrected for selection effects. The strengths of these correlations are very similar in both populations, implying similar mechanisms at work in both GRB classes, leading to the emergence of these prompt gamma-ray correlations.

GRB 130603B: No Compelling Evidence for Neutron Star Merger

The near infrared (NIR) flare/rebrightening in the afterglow of the short hard gamma ray burst (SHB) 130603B measured with the Hubble Space Telescope (HST) and an alleged late-time X-ray excess were interpreted as possible evidence of a neutron star merger origin of SHBs. However, the X-ray afterglow that was measured with the Swift XRT and Newton XMM has the canonical behaviour of a synchrotron afterglow produced by a highly relativistic jet. The H-band flux observed with HST 9.41 days after burst is that expected from the measured late-time X-ray afterglow. The late-time flare/rebrightening of the NIR-optical afterglow of SHB 130603B could have been produced also by jet collision with an interstellar density bump. Moreover, SHB plus a kilonova can be produced also by the collapse of a compact star (neutron star, strange star, or quark star) to a more compact object due to cooling, loss of angular momentum, or mass accretion.

First spectroscopy of a short-hard GRB: the environment of a compact object merger

AbstractShort gamma-ray bursts (GRBs) are an extremely elusive family of cosmic explosions. They are thought to be related to the violent merger of compact objects (such as a neutron stars or black holes). Their optical counterparts were not discovered until 2005, and since then, there had been no successful spectroscopic observations. Here we present the first spectra of a short GRB, which we use to study the environment and derive implications on the progenitors of these cosmic explosions. This poster is based on the work by de Ugarte Postigoet al. (2014).

Search for gravitational waves associated with γ-ray bursts detected by the interplanetary network.

We present the results of a search for gravitational waves associated with 223 γ-ray bursts (GRBs) detected by the InterPlanetary Network (IPN) in 2005-2010 during LIGO's fifth and sixth science runs and Virgo's first, second, and third science runs. The IPN satellites provide accurate times of the bursts and sky localizations that vary significantly from degree scale to hundreds of square degrees. We search for both a well-modeled binary coalescence signal, the favored progenitor model for short GRBs, and for generic, unmodeled gravitational wave bursts. Both searches use the event time and sky localization to improve the gravitational wave search sensitivity as compared to corresponding all-time, all-sky searches. We find no evidence of a gravitational wave signal associated with any of the IPN GRBs in the sample, nor do we find evidence for a population of weak gravitational wave signals associated with the GRBs. For all IPN-detected GRBs, for which a sufficient duration of quality gravitational wave data are available, we place lower bounds on the distance to the source in accordance with an optimistic assumption of gravitational wave emission energy of 10(-2)M⊙c(2) at 150 Hz, and find a median of 13 Mpc. For the 27 short-hard GRBs we place 90% confidence exclusion distances to two source models: a binary neutron star coalescence, with a median distance of 12 Mpc, or the coalescence of a neutron star and black hole, with a median distance of 22 Mpc. Finally, we combine this search with previously published results to provide a population statement for GRB searches in first-generation LIGO and Virgo gravitational wave detectors and a resulting examination of prospects for the advanced gravitational wave detectors.