Wide angle x-ray sky monitoring for corroborating non-electromagnetic cosmic transients

Gravitational waves (GWs) can be emitted from coalescing neutron star (NS) and black hole-neutron star 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 are expected to be observationally coincident with SHBs. Similarly, ultrahigh energy neutrino signals associated with gamma-ray bursts could fail to be corroborated by prompt γ-ray emission if the latter is beamed into a narrower cone than the neutrinos. Alternative ways to corroborate non-electromagnetic signals from coalescing NSs are therefore all the more desirable. It is noted here that the extended X-ray tails (XRTs) 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 γ-rays, thereby enhancing the possibility of detecting it as a GW or neutrino source. Scattered γ-rays, which may subtend a much larger solid angle than the primary gamma-ray jet, are also candidates for corroborating non-electromagnetic signals.