X-ray Transients in the Advanced LIGO/Virgo Horizon

TL;DR

This study systematically measured the rate of extragalactic soft X-ray transients within the Advanced LIGO/Virgo detection horizon, identifying potential counterparts to gravitational wave events, including tidal disruption events.

Contribution

First extragalactic measurement of soft X-ray transient rate within the LIGO/Virgo horizon, identifying candidate tidal disruption events and assessing confusion risks for gravitational wave counterparts.

Findings

Transients comprise 10% of variable objects in the survey.

Detected a transient rate of 4x10^-4 per square degree above a certain flux.

Identified six transients with properties consistent with tidal disruption events.

Abstract

Advanced LIGO and Advanced Virgo will be all-sky monitors for merging compact objects within a few hundred Mpc. Finding the electromagnetic counterparts to these events will require an understanding of the transient sky at low red-shift (z<0.1). We performed a systematic search for extragalactic, low red-shift, transient events in the XMM-Newton Slew Survey. In a flux limited sample, we found that highly-variable objects comprised 10% of the sample, and that of these, 10% were spatially coincident with cataloged optical galaxies. This led to 4x10^-4 transients per square degree above a flux threshold of 3x10^-12 erg cm-2 s-1 [0.2-2 keV] which might be confused with LIGO/Virgo counterparts. This represents the first extragalactic measurement of the soft X-ray transient rate within the Advanced LIGO/Virgo horizon. Our search revealed six objects that were spatially coincident with previously cataloged galaxies, lacked evidence for optical AGNs, displayed high luminosities around 10^43 erg s-1, and varied in flux by more than a factor of ten when compared with the ROSAT All-Sky Survey. At least four of these displayed properties consistent with previously observed tidal disruption events.