SOFT X-RAY EXTENDED EMISSIONS OF SHORT GAMMA-RAY BURSTS AS ELECTROMAGNETIC COUNTERPARTS OF COMPACT BINARY MERGERS: POSSIBLE ORIGIN AND DETECTABILITY

摘要

We investigate the possible origin of extended emissions (EEs) of short gamma-ray bursts with an isotropic energy of ~1050-51 erg and a duration of a few 10 s to ~100 s, based on a compact binary (neutron star (NS)-NS or NS-black hole (BH)) merger scenario. We analyze the evolution of magnetized neutrino-dominated accretion disks of mass ~0.1 M ☉ around BHs formed after the mergers and estimate the power of relativistic outflows via the Blandford-Znajek (BZ) process. We show that a rotation energy of the BH up to 1052 erg can be extracted with an observed timescale of 30(1 + z) s with a relatively small disk viscosity parameter of α 0.01. Such a BZ power dissipates by clashing with non-relativistic pre-ejected matter of mass M ~ 10–(2-4) M ☉, and forms a mildly relativistic fireball. We show that the dissipative photospheric emissions from such fireballs are likely in the soft X-ray band (1-10 keV) for M ~ 10–2 M ☉, possibly in NS-NS mergers, and in the BAT band (15-150 keV) for M ~ 10–4 M ☉, possibly in NS-BH mergers. In the former case, such soft EEs can provide a good chance of for simultaneous detections of the gravitational waves with a ~01 angular resolution by soft X-ray survey facilities like the Wide-Field MAXI. Here, ΔΩsoftEE is the beaming factor of the soft EEs and is the NS-NS merger rate detectable by the advanced LIGO, the advanced Virgo, and KAGRA.