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
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 deg2 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 ≈\,21 mAB 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