GCN Circular 27330

Y.-L. Mong (1); K. Ackley (1); D. K. Galloway (1); D. Steeghs (2); V.
Dhillon (3); P. O'Brien (4); G. Ramsay (5); D. Pollacco (2); E. Thrane (1);
S. Poshyachinda (6); R. Kotak (7); L. Nuttall (8); E. Pall\'e (9); K.
Ulaczyk (2); J. Lyman (2); R. Cutter (2); A. Levan (2); T. Marsh (2); R.
West (2); E. Stanway (2); B. Gompertz (2); K. Wiersema (2); T. Killestein
(2); A. Casey (1); M. Brown (1); B. Muller (1); M. Dyer (3); J. Mullaney
(3); E. Daw (3); S. Littlefair (3); J. Maund (3); L. Makrygianni (3); U.
Burhanudin (3); R. Starling (4); R. Eyles (4); S. Tooke (4); S.
Aukkaravittayapun (6); U. Sawangwit (6); S. Awiphan (6); D. Mkrtichian (6);
P. Irawati (6); S. Mattila (7); T. Heikkil\"a (7); E. Rol (1)

We report on optical observations with the Gravitational-wave Optical
Transient Observer prototype in response to S200302c (GCN #27278).
Targeted observations started shortly after the preliminary event
notification was received. These spanned 102 unique tile pointings
containing 30.8% of the source location probability (based on the initial
BAYESTAR skymap) and were acquired between 20:15 UT Mar 02 2020 and
20:35 UT Mar 04 2020 (starting 18.3 hours after the event trigger time
01:58 UT Mar 02 2020). No new transients that could be credibly
associated with S200302c were detected.

Each pointing spans 4.9x3.7 square degrees and consisted of 3x60s
exposures in our L-band filter (400-700nm passband similar to g+r)
with a median 5-sigma photometric depth equivalent to g=19.1 for an
individual pointing. Limits are based on a photometric calibration
against PS1 sources. Most pointings were observed multiple times,
typically 2-3 times.

Images are processed immediately after acquisition using the GOTOphoto
pipeline. Difference imaging was performed on the median of each triplet
of exposures using recent survey observations of the same pointings.
Source candidates were initially filtered using a classifier and
cross-matched against a variety of catalogs, including the MPC and PS1.
Human candidate vetting was performed following data acquisition and
automated classifier cuts.

GOTO is operated at the La Palma observing facilities of the University
of Warwick on behalf of a consortium including the University of
Warwick, Monash University, Armagh Observatory, the University of
Leicester, the University of Sheffield, the National Astronomical
Research Institute of Thailand (NARIT), The University of Portsmouth,
the University of Turku and the Instituto de Astrofisica de Canarias
(IAC) (https://goto-observatory.org/)