Swift-XRT Follow-up of Gravitational-wave Triggers in the Second Advanced LIGO/Virgo Observing Run
The Neil Gehrels Swift Observatory carried out prompt searches for gravitational-wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW170814 and the epochal GW170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2 and how these are being used to improve the Swift follow-up of GW events. In particular, we simulate a population of gamma-ray burst afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncataloged X-ray sources. We find that ≈60%–70% of afterglows whose jets are oriented toward Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming that their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.
Additional Information© 2019 The American Astronomical Society. Received 2019 August 20; revised 2019 October 10; accepted 2019 October 11; published 2019 November 15. N.J.K. would like to acknowledge support from NASA grant 80NSSC19K0408. P.A.E., A.P.B., J.P.O., and K.L.P. acknowledge support from the UK Space Agency. S.R.O. gratefully acknowledges the support of the Leverhulme Trust Early Career Fellowship. The Dark Cosmology Centre was funded by the Danish National Research Foundation. D.B.M. is supported by research grant 19054 from Villum Fonden. A.D. acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0. The authors would also like to thank the anonymous referee for their useful suggestions, which helped to improve the paper. Facility: the Neil Gehrels Swift Observatory. - Software: HEAsoft (v6.22; HEASARC 2014), afterglowpy (v0.6.4; Ryan et al. 2019).
Published - Klingler_2019_ApJS_245_15.pdf
Accepted Version - 1909.11586.pdf