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

We investigate the possible origin of extended emissions (EEs) of short gamma-ray bursts with an isotropic energy of similar to 10(50-51) erg and a duration of a few 10 s to similar 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 similar to 0.1 M-circle dot 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 greater than or similar to 10(52) erg can be extracted with an observed timescale of greater than or similar to 30(1 + z) s with a relatively small disk viscosity parameter of alpha < 0.01. Such a BZ power dissipates by clashing with non-relativistic pre-ejected matter of mass M similar to 10(-(2-4)) M-circle dot, 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 similar to 10(-2) M-circle dot, possibly in NS-NS mergers, and in the BAT band (15-150 keV) for M similar to 10(-4) M-circle dot, possibly in NS-BH mergers. In the former case, such soft EEs can provide a good chance of similar to 6 yr(-1) (Delta Omega E-softEE/4 pi) (R-GW/40 yr(-1)) for simultaneous detections of the gravitational waves with a similar to 0 degrees.1 angular resolution by soft X-ray survey facilities like the Wide-Field MAXI. Here, Delta Omega(softEE) is the beaming factor of the soft EEs and R-GW is the NS-NS merger rate detectable by the advanced LIGO, the advanced Virgo, and KAGRA.