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Register a new userShort GRBs: opening angles, local neutron star merger rate and off-axis events for GRB/GW association
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The jet breaks in the afterglow lightcurves of short gamma-ray bursts (SGRBs), rarely detected so far, are crucial for estimating the half-opening angles of the ejecta ($theta_{rm j}$) and hence the neutron star merger rate. In this work we report the detection of jet decline behaviors in GRB 150424A and GRB 160821B and find $theta_{rm j}sim 0.1$ rad. Together with five events reported before 2015 and other three identified recently (GRB 050709, GRB 060614 and GRB 140903A), we have a sample consisting of nine SGRBs and one long-short GRB with reasonably estimated $theta_{rm j}$. In particular, three {it Swift} bursts in the sample have redshifts $zleq 0.2$, with which we estimate the local neutron star merger rate density {to be $sim 1109^{+1432}_{-657}~{rm Gpc^{-3}~yr^{-1}}$ or $162^{+140}_{-83} {rm Gpc^{-3}yr^{-1}}$ if the narrowly-beamed GRB 061201 is excluded}. Inspired by the typical $theta_{rm j}sim 0.1$ rad found currently, we further investigate whether the off-beam GRBs (in the uniform jet model) or the off-axis events (in the structured jet model) can significantly enhance the GRB/GW association or not. For the former the enhancement is at most moderate, while for the latter the enhancement can be much greater and a high GRB/GW association probability of $sim 10%$ is possible. We also show that the data of GRB 160821B may contain a macronova/kilonova emission component with a temperature of $sim 3100$ K at $sim 3.6$ days after the burst and more data are needed to ultimately clarify.
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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 $Delta tapprox1.74;$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 ($E_{rmgamma,iso}approx5times10^{46};$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 jets 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 $Esim10^{48.5}-10^{49.5}~{rm erg}$, and external density $nsim10^{-5}-10^{-1}~{rm cm}^{-3}$ for a $xi_esim 0.1$ non-thermal electron acceleration efficiency.
The recent radio observations (Mooley et al, 2018) of a superluminal radio afterglow following GRB 170817A 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 {bf at a viewing angle of $sim 0.2$ radians from the emitting material (perhaps slightly more from jet axis)}, we suggest that the Lorentz factor of the jet is $lesssim 20$ 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 (Fong, et al., 2016). A feature of GRBs viewed from large offset angles is a large afterglow isotropic equivalent energy as compared to prompt emission, as predicted (Eichler, 2017), 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 ($theta gg 1/Gamma $), could not be made by internal shocks in the baryonic material that powers the afterglow.
We recently found that Gamma Ray Burst energies and luminosities, in their comoving frame, are remarkably similar. This, coupled with the clustering of energetics once corrected for the collimation factor, suggests the possibility that all bursts, in their comoving frame, have the same peak energy Epeak (of the order of a few keV) and the same energetics of the prompt emission Egamma (of the order of 2e48 erg). The large diversity of bursts energies is then due to the different bulk Lorentz factor Gamma and jet aperture angle theta_jet. We investigated, through a population synthesis code, what are the distributions of Gamma and theta_jet compatible with the observations. Both quantities must have preferred values, with log-normal best fitting distributions and <Gamma0> ~ 275 and <theta_jet> ~ 8.7 degree. Moreover, the peak values of the Gamma and theta_jet distributions must be related - theta_jet^2.5 Gamma =const: the narrower the jet angle, the larger the bulk Lorentz factor. We predict that ~6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin(theta_jet)<1/Gamma. Finally, we estimate that the local rate of GRBs is ~0.3% of all local SNIb/c and ~2.5% of local hypernovae, i.e. SNIb/c with broad absorption lines.
VLBI and JVLA observations revealed that GW170817 involved a narrow jet ($ theta_j approx 4^circ $) that dominated the afterglow peak at our viewing angle, $ theta_{rm obs} approx 20^circ $. This implies that at the time of the afterglow peak, the observed signal behaved like an afterglow of a top-hat jet seen at $ theta_{rm obs} gg theta_j $, and it can be modeled by analytic expressions that describe such jets. We use a set of numerical simulations to calibrate these analytic relations and obtain generic equations for the peak time and flux of such an afterglow as seen from various observing angles. Using the calibrated equations and the estimated parameters of GW170817, we estimate the detectability of afterglows from future double neutron star mergers during the Advanced LIGO/Virgo observation run O3. GW170817 took place at a relatively low-density environment. Afterglows of similar events will be detectable only at small viewing angles, $ theta_{rm obs} lesssim 20^circ $, and only $sim 20% $ of the GW detections of these events will be accompanied by a detectable afterglow. At higher densities, more typical to sGRB sites, up to $ 70% $ of the GW detections are expected to be followed by a detectable afterglow, typically at $ theta_{rm obs} sim 30^circ $. We also provide the latest time one should expect an afterglow detection. We find that for typical parameters, if the jet emission had not been detected within about a year after the merger, it is unlikely to be ever detected.
Gravitational waves from coalescence of a Binary Neutron Star (BNS) and its accompagning short Gamma-Ray Burst GW/GRB~170817A confirmed the presumed origin of these puzzeling transients and opened up the way for relating properties of short GRBs to those of their progenitor stars and their surroundings. Here we review an extensive analysis of the prompt gamma-ray and late afterglows of this event. We show that a fraction of polar ejecta from the merger had been accelerated to ultra-relativistic speeds. This structured jet had an initial Lorentz factor of about $260$ in our direction - $mathcal{O}(10^circ)$ from the jets axis - and was a few orders of magnitude less dense than in typical short GRBs. At the time of arrival to circum-burst material the ultra-relativistic jet had a close to Gaussian profile and a Lorentz factor $gtrsim 130$ in its core. It had retained in some extent its internal collimation and coherence, but had extended laterally to create mildly relativistic lobes - a {it cocoon}. External shocks on the far from center inhomogeneous circum-burst material and low density of colliding shells generated slow rising afterglows. The circum-burst material was somehow correlated with the merger and it is possible that it contained recently ejected material from glitching, which had resumed due to the deformation of neutron stars crust by tidal forces in the latest stages of inspiral but well before their merger. By comparing these findings with the results of relativistic MHD simulations and observed gravitational waves we conclude that progenitor neutron stars were old, had close masses and highly reduced magnetic fields. In addition, they probably had oppositely directed spins due to the encounter and gravitational interaction with other stars.
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