Seeking Counterparts to Advanced LIGO/Virgo Transients with Swift

TL;DR

This paper explores how the Swift satellite can efficiently detect X-ray counterparts to gravitational wave events from neutron star mergers, enhancing multi-messenger astronomy.

Contribution

It proposes two effective search strategies using Swift XRT to identify X-ray afterglows associated with LIGO/Virgo gravitational wave triggers.

Findings

Swift XRT can detect X-ray afterglows brighter than 10^-12 erg/s/cm^2.

Search strategies can cover ~100 Mpc volume and 35 square degrees within a day.

Detection of X-ray counterparts to GW signals is feasible with proposed methods.

Abstract

Binary neutron star (NS) mergers are among the most promising astrophysical sources of gravitational wave emission for Advanced LIGO and Advanced Virgo, expected to be operational in 2015. Finding electromagnetic counterparts to these signals will be essential to placing them in an astronomical context. The Swift satellite carries a sensitive X-ray telescope (XRT), and can respond to target-of-opportunity requests within 1-2 hours, and so is uniquely poised to find the X-ray counterparts to LIGO/Virgo triggers. Assuming NS mergers are the progenitors of short gamma-ray bursts (GRBs), some percentage of LIGO/Virgo triggers will be accompanied by X-ray band afterglows that are brighter than 10^-12 erg/s/cm^2 in the XRT band one day after the trigger time. We find that a soft X-ray transient of this flux is bright enough to be extremely rare, and so could be confidently associated with even a moderately localized GW signal. We examine two possible search strategies with the Swift XRT to find bright transients in LIGO/Virgo error boxes. In the first strategy, XRT could search a volume of space with a ~100 Mpc radius by observing ~30 galaxies over the course of a day, with sufficient depth to observe the expected X-ray afterglow. For an extended LIGO/Virgo horizon distance, the XRT could employ 100 s exposures to cover an area of ~ 35 square degrees in about a day, remain sensitive enough to image GW discovered GRB afterglows. These strategies demonstrate that discovery of X-ray band counterparts to GW triggers will be possible.