Abstract
Precursor emissions are found in some short gamma-ray bursts (SGRBs). In this paper, we review the theories and observations of the SGRB precursor and discuss its prospect as an electromagnetic counterpart of the gravitational wave event produced by neutron star (NS) mergers. The observed luminosity, spectrum, and duration of precursors are explained by the magnetospheric interaction model during the inspiral or the cocoon/jet shock breakout model during the jet propagation. In general, these two models predict that the precursor will be weaker than the main GRB, but will be of a larger opening angle, which makes it an advantageous gamma-ray counterpart for NS mergers in the local Universe, especially for NS - black hole mergers with very low mass ratios, in which the main GRBs are not expected. The joint observation of the precursor, SGRB, and gravitational wave will help to reveal the jet launch mechanism and post-merger remnant.
Highlights
On 17 August 2017, the Fermi/gamma-ray burst monitor (GBM) was triggered by a short gamma-ray burst (SGRB)-GRB 170817A [1,2,3]
Precursors are found in both long GRBs and SGRBs, and the detection rate of precursor is higher in long GRBs (e.g., [35,81])
Precursors have been detected for a small fraction of SGRBs
Summary
On 17 August 2017, the Fermi/gamma-ray burst monitor (GBM) was triggered by a short gamma-ray burst (SGRB)-GRB 170817A [1,2,3]. The joint detection of GW170817/GRB 170817A confirms that at least some SGRBs originate from NS mergers, and herald the multi-messenger astronomy [1,2,3,4,5]. It enables better localization, which benefited the multi-wavelength follow-up observations. Joint detection can provide abundant information to study some fundamental physics. Combining with the electromagnetic (EM) observations, (1) the GW event can be treated as a standard siren to study cosmology [12]; (2) one can constrain the difference between the speed of gravity and the light speed, test the violation of Lorentz invariance and the equivalence principle [5,13]. It can be used to study the launching mechanism, structure, composition, and radiation mechanism of GRB jets (e.g., [3,14,15,16,17,18,19,20,21,22,23])
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