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Register a new userGamma-ray burst models in light of the GRB 170817A - GW170817 connection
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For the first time, a short gamma-ray burst (GRB) was unambiguously associated with a gravitational wave (GW) observation from a binary neutron star (NS) merger. This allows us to link the details of the central engine properties to GRB emission models. We find that photospheric models (both dissipative and non-dissipative variants) have difficulties accounting for the observations. Internal shocks give the most natural account of the observed peak energy, viewing angle and total energy. We also show that a simple external shock model can reproduce the observed GRB pulse with parameters consistent with those derived from the afterglow modeling. We find a simple cocoon shock breakout model is in mild tension with the observed spectral evolution, however it cannot be excluded based on gamma-ray data alone. Future joint observations of brighter GRBs will pose even tighter constraints on prompt emission models.
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A preponderance of evidence links long-duration, soft-spectrum gamma-ray bursts (GRBs) with the death of massive stars. The observations of the GRB-supernova (SN) connection present the most direct evidence of this physical link. We summarize 30 GRB-SN associations and focus on five ironclad cases, highlighting the subsequent insight into the progenitors enabled by detailed observations. We also address the SN association (or lack thereof) with several sub-classes of GRBs, finding that the X-ray Flash (XRF) population is likely associated with massive stellar death whereas short-duration events likely arise from an older population not readily capable of producing a SN concurrent with a GRB. Interestingly, a minority population of seemingly long-duration, soft-spectrum GRBs show no evidence for SN-like activity; this may be a natural consequence of the range of Ni-56 production expected in stellar deaths.
In the faint short gamma-ray burst sGRB 170817A followed by the gravitational waves (GWs) from a merger of two neutron stars (NSs) GW170817, the spectral peak energy is too high to explain only by canonical off-axis emission. We investigate off-axis appearance of an sGRB prompt emission scattered by a cocoon, which is produced through the jet-merger-ejecta interaction, with either sub-relativistic or mildly-relativistic velocities. We show that the observed properties of sGRB 170817A, in particular the high peak energy, can be consistently explained by the Thomson-scattered emission with a typical sGRB jet, together by its canonical off-axis emission, supporting that an NS-NS merger is the origin of sGRBs. The scattering occurs at $lesssim 10^{10}$--$10^{12},{rm cm}$ not far from the central engine, implying the photospheric or internal shock origin of the sGRB prompt emission. The boundary between the jet and cocoon is sharp, which could be probed by future observations of off-axis afterglows. The scattering model predicts a distribution of the spectral peak energy that is similar to the observed one but with a cutoff around $sim$ MeV energy, and its correlations with the luminosity, duration, and time lag from GWs, providing a way to distinguish it from alternative models.
Binary neutron-star mergers (BNSMs) are among the most readily detectable gravitational-wave (GW) sources with LIGO. They are also thought to produce short $gamma$-ray bursts (SGRBs), and kilonovae that are powered by r-process nuclei. Detecting these phenomena simultaneously would provide an unprecedented view of the physics during and after the merger of two compact objects. Such a Rosetta Stone event was detected by LIGO/Virgo on 17 August 2017 at a distance of $sim 44$ Mpc. We monitored the position of the BNSM with ALMA at 338.5 GHz and GMRT at 1.4 GHz, from 1.4 to 44 days after the merger. Our observations rule out any afterglow more luminous than $3times 10^{26}~{rm erg,s}^{-1},{rm Hz}^{-1}$ in these bands, probing $>$2--4 dex fainter than previous SGRB limits. We match these limits, in conjunction with public data announcing the appearance of X-ray and radio emission in the weeks after the GW event, to templates of off-axis afterglows. Our broadband modeling suggests that GW170817 was accompanied by a SGRB and that the GRB jet, powered by $E_{rm AG,,iso}sim10^{50}$~erg, had a half-opening angle of $sim20^circ$, and was misaligned by $sim41^circ$ from our line of sight. The data are also consistent with a more collimated jet: $E_{rm AG,,iso}sim10^{51}$~erg, $theta_{1/2,,rm jet}sim5^circ$, $theta_{rm obs}sim17^circ$. This is the most conclusive detection of an off-axis GRB afterglow and the first associated with a BNSM-GW event to date. Assuming a uniform top-hat jet, we use the viewing angle estimates to infer the initial bulk Lorentz factor and true energy release of the burst.
Motivating by the discovery of association between GW 170817 and sGRB 170817A, we present a comprehensive analysis for sGRBs observed with Fermi/GBM in 9 operation years and study the properties of sGRB 170817A -like events. We derive a catalog of 275 typical sGRBs and 48 sGRB 170817A-like weak events from the GBM data of 2217 GRBs. We visibly identify two patterns of their light curve, single episode (Pattern I, 61% of the SGRBs) and multiple episodes (Pattern II, 39% of the SGRBs). Their duration distribution shows a tentative bimodal feature. Their spectra can be fitted with a cutoff power-law model, except for 4 sGRBs, and the spectral indices normally distribute at $Gamma=0.69pm 0.40$. Their $E_p$ values show a tentative bimodal distribution with peaks at 145 keV and 500 keV. No correlation among $T_{90}$, $E_p$, and $Gamma$ is found. GRB 170817A is a soft, weak sGRB with $ E_{p}=124pm 106$ keV, $L_{rm iso}=(5.67pm4.65)times10^{46}rm ~erg~s^{-1}$, and $E_{rm iso}=(3.23pm2.65)times10^{46}rm ~erg$. It follows the $E_{rm iso}-E_{rm p}$ relation of typical short GRBs. Its lightcurve is of Pattern II. Two lightcurve patterns, together with the potential two components in the $E_{rm p}$ and $T_{90}$ distributions, we suspect that the current sample may include two distinct types of sGRBs from different progenitors. sGRB 170817A-like events may be from NS-NS mergers and those sGRBs with a Pattern I lightcurve may be from another distinct type of compact binary.
The observed association between supernovae and gamma-ray bursts represents a cornerstone in our understanding of the nature of gamma-ray bursts. The collapsar model provides a theoretical framework for this connection. A key element is the launch of a bi-polar jet (seen as a gamma-ray burst). The resulting hot cocoon disrupts the star while the 56Ni produced gives rise to radioactive heating of the ejecta, seen as a supernova. In this discussion paper I summarise the observational status of the supernova/gamma-ray burst connection in the context of the engine picture of jet-driven supernovae and highlight SN 2012bz/GRB 120422A -- with its luminous supernova but intermediate high-energy luminosity -- as a possible transition object between low-luminosity and jet gamma-ray bursts. The jet channel for supernova explosions may provide new insight into supernova explosions in general.
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