Swift-XRT Follow-up of Gravitational-wave Triggers in the Second Advanced LIGO/Virgo Observing Run

N. J. Klingler*, J. A. Kennea, P. A. Evans, A. Tohuvavohu, S. B. Cenko, S. D. Barthelmy, A. P. Beardmore, E. Troja, P. J. Brown, D. N. Burrows, S. Campana, G. Cusumano, A. D'Ai, P. D'Avanzo, V D'Elia, M. de Pasquale, S. W. K. Emery, J. Garcia, P. Giommi, C. GronwallD. H. Hartmann, H. A. Krimm, N. P. M. Kuin, A. Lien, D. B. Malesani, F. E. Marshall, A. Melandri, J. A. Nousek, S. R. Oates, P. T. O'Brien, J. P. Osborne, K. L. Page, D. M. Palmer, M. Perri, J. L. Racusin, M. H. Siegel, T. Sakamoto, B. Sbarufatti, G. Tagliaferri, E. Troja

*Corresponding author for this work

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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.
Original languageEnglish
Article number15
JournalAstrophysical Journal Supplement Series
Issue number1
Number of pages14
Publication statusPublished - 2019

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