GROWTH on GW190425: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility and Palomar Gattini IR

Abstract : The beginning of the third observing run by the network of gravitational-wave detectors has brought the discovery of many compact binary coalescences. Prompted by the detection of the first binary neutron star merger in this run (GW190425 / LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. As it was a single gravitational-wave detector discovery, the initial skymap spanned most of the sky observable from Palomar Observatory, the site of both instruments. Covering 8000 deg$^2$ of the inner 99% of the initial skymap over the next two nights, corresponding to an integrated probability of 46%, the ZTF system achieved a depth of $\approx$ 21 $m_\textrm{AB}$ in $g$- and $r$-bands. Palomar Gattini-IR covered a total of 2200 square degrees in $J$-band to a depth of 15.5 mag, including 32% of the integrated probability based on the initial sky map. However, the revised skymap issued the following day reduced these numbers to 21% for the Zwicky Transient Facility and 19% for Palomar Gattini-IR. Out of the 338,646 ZTF transient "alerts" over the first two nights of observations, we narrowed this list to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod were particularly compelling given that their location, distance, and age were consistent with the gravitational-wave event, and their early optical lightcurves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were photometrically or spectroscopically ruled-out as supernovae. Palomar Gattini-IR identified one fast evolving infrared transient after the merger, PGIR19bn, which was later spectroscopically classified as an M-dwarf flare. [abridged]
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https://hal.archives-ouvertes.fr/hal-02283504
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Submitted on : Tuesday, September 10, 2019 - 10:10:58 PM
Last modification on : Tuesday, September 17, 2019 - 10:07:50 PM

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Michael W. Coughlin, Tomas Ahumada, Shreya Anand, Kishalay De, Matthew J. Hankins, et al.. GROWTH on GW190425: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility and Palomar Gattini IR. 2019. ⟨hal-02283504⟩

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