Wide Angle X-ray Sky Monitoring for Corroborating non-Electromagnetic Cosmic Transients

TL;DR

This paper proposes using wide-angle X-ray sky monitoring to improve the detection and corroboration of non-electromagnetic signals like gravitational waves and neutrinos from neutron star mergers, which are often missed by narrow gamma-ray beaming.

Contribution

It introduces the idea that X-ray tails and flashes can serve as wider-angle electromagnetic counterparts to gravitational wave events, increasing detection chances.

Findings

X-ray tails resemble X-ray flashes, indicating off-axis viewing angles.

Wider solid angle emission in X-rays can enhance GW and neutrino detection.

Scattered gamma-rays may also serve as broader electromagnetic counterparts.

Abstract

Gravitational waves (GW) can be emitted from coalescing neutron star (NS) and black hole-neutron star (BH-NS) binaries, which are thought to be the sources of short hard gamma ray bursts (SHBs). The gamma ray fireballs seem to be beamed into a small solid angle and therefore only a fraction of detectable GW events is expected to be observationally coincident with SHBs. Similarly ultrahigh energy (UHE) neutrino signals associated with gamma ray bursts (GRBs) could fail to be corroborated by prompt gamma-ray emission if the latter is beamed in a narrower cone than the neutrinos. Alternative ways to corroborate non-electromagnetic signals from coalescing neutron stars are therefore all the more desirable. It is noted here that the extended X-ray tails (XRT) of SHBs are similar to X-ray flashes (XRFs), and that both can be attributed to an off-axis line of sight and thus span a larger solid angle than the hard emission. It is proposed that a higher fraction of detectable GW events may be coincident with XRF/XRT than with hard gamma-rays, thereby enhancing the possibility to detect it as a GW or neutrino source. Scattered gamma-rays, which may subtend a much larger solid angle that the primary gamma ray jet, are also candidates for corroborating non-electromagnetic signals.