Short GRBs: Opening Angles, Local Neutron Star Merger Rate, and Off-axis Events for GRB/GW Association

Abstract

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 (θ 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 θ j ∼ 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 θ j . In particular, three Swift bursts in the sample have redshifts z ≤ 0.2, with which we estimate the local neutron star merger rate density to be ∼ 1109 +1432 −657 Gpc −3 yr −1 or 162 +140 −83 Gpc −3 yr −1 if the narrowly-beamed GRB 061201 is excluded. Inspired by the typical θ j ∼ 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 ∼ 10% is possible. We also show that the data of GRB 160821B may contain a macronova/kilonova emission component with a temperature of ∼ 3100 K at ∼ 3.6 days after the burst and more data are needed to ultimately clarify.

The afterglow and kilonova of the short GRB 160821B

“…The short duration GRB160821B, thanks to its low redshift and a rich multi-wavelength dataset, is an excellent testbed for kilonova searches and afterglow studies. The presence of a kilonova was discussed by Kasliwal et al (2017a) and Jin et al (2018), who did not find conclusive evidence for it due to the sparse dataset used in both studies. Here we present a comprehensive broadband analysis of this event, including data from Swift and XMM-Newton in the X-rays, the Gran Telescopio de Canarias (GTC), the William Herschel Telescope (WHT), the Hubble Space Telescope (HST), the Keck I telescope, and the Discovery Channel Telescope (DCT) in the optical/nIR, and the Jansky Very Large Array (VLA) in the radio.…”

“…XMM-Newton data rule out the presence of an early ( 1 d) temporal break, proposed by Jin et al (2018) based on a smaller dataset. They are instead consistent with an uninterrupted power-law decay up to ≈3.5 d and no spectral evolution.…”

Short GRB 160821B: A Reverse Shock, a Refreshed Shock, and a Well-sampled Kilonova

“…These observations indicate an excess in blue at early times followed by a reddening; this behaviour is indicative of a kilonova. Previous studies of sGRB 160821B have been restricted to much smaller photometric data-sets and consequently have only drawn weak conclusions about the possibility of a kilonova component and the nature of the afterglow (Jin et al 2018;Kasliwal et al 2017b;Gompertz et al 2018). Here, we use the X-ray, early optical and radio constraints on the afterglow emission to interpret the kilonova contribution at optical and near-IR frequencies.…”

Binary neutron star mergers with missing electromagnetic counterparts as manifestations of mirror world

“…Let us address to short gamma ray burst (sGRB) phenomena which are believed to be prompt electromagnetic signals produced by BNS and BH-NS mergers [16]. sGRBs are called the GRBs with duration less than 2 seconds detected at the Earth orbit at rate 1109 +1432 −657 Gpc −3 yr −1 [17]. From the other side, BNS merging rate can be estimated using gravitational waves signals.…”