GCN Circular 27317

K. Ackley (1); Y.-L. Mong (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)

((1) Monash University, (2) Warwick University, (3) University of 
Sheffield, (4) University of Leicester, (5) Armagh Observatory & 
Planetarium, (6) National Astronomical Research Institute of Thailand, 
(7) University of Turku, (8) University of Portsmouth, (9) Instituto de 
Astrofisica de Canarias)

report on behalf of the GOTO collaboration:

We report on optical observations with the Gravitational-wave Optical 
Transient Observer prototype in response to S200225q (GCN #27193). 
Targeted observations started shortly after the preliminary event 
notification was received. These spanned 64 unique tile pointings, with 
image subtraction, containing 81.6% of the source location probability 
(based on the initial BAYESTAR skymap) and were acquired between 21:46 
UT Feb 25 2020 and 00:30 UT Feb 26 2020 (starting 15.6 hours after the 
event trigger). No new transients that could be credibly associated with 
S200225q 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=19.3 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/)