TITLE:   GCN CIRCULAR
NUMBER:  28159
SUBJECT: GRB 200723A: GOTO candidate afterglow and host galaxy
DATE:    20/07/24 10:33:09 GMT
FROM:    Ben Gompertz at U of Warwick  <b.gompertz@warwick.ac.uk>

B. Gompertz (1); K. Wiersema (1); D. K. Galloway (2); R. Starling (3); 
J. Lyman (1); K. Ackley (2); Y.-L. Mong (2); D. Steeghs (1); V.Dhillon 
(4); P. O'Brien (3); G. Ramsay (5); D. Pollacco (1); E. Thrane (2); S. 
Poshyachinda (6); R. Kotak (7); L. Nuttall (8); E. Pall\'e (9); K. 
Ulaczyk (1); R. Cutter (1); A. Levan (1); T. Marsh (1); R. West (1); E. 
Stanway (1); T. Killestein (1); A. Casey (2); M. Brown (2); B. Muller 
(2); M. Dyer (4); J. Mullaney (4); E. Daw (4); S. Littlefair (4); J. 
Maund (4); L. Makrygianni (4); U. Burhanudin (4); R. Eyles (3); S. Tooke 
(3); 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) Warwick University, (2) Monash University, (3) University of 
Leicester, (4) University of Sheffield, (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 carried out observations with the Gravitational-wave Optical 
Transient Observer (GOTO) in response to GRB 200723A (Fermi GBM; GCN 28155).

We made a series of 4x90 s exposures using our wide L-band filter 
(400-700 nm) covering the Fermi GBM error box (GCN 28155), with midtime 
22:42:55 UT on 23 July 2020.

Using a difference imaging analysis with recent survey observations of 
the same pointings as reference, we detect an uncatalogued source in a 
tiled observation taken at 21:25 UT (4.8 hours after trigger) with a 
magnitude of L = 18.5 ± 0.05 mag based on PS1 catalogue calibrators 
located at (J2000):
RA 16:18:34.973
Dec +60:19:16.21

This source is coincident with a galaxy in the 2MASS redshift survey 
(Huchra et al, 2012) at a redshift of z = 0.0557. Given the large Fermi 
GBM uncertainty region, the probability that this source is an unrelated 
transient is large. We have registered it with the TNS with the ID 
AT2020qer. A previous GOTO observation taken on July 17 23:28:49 UT 
places a limiting magnitude of L > 19.2 (5 sigma) on this source 6 days 
prior to the Fermi GBM trigger.

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) and the Instituto de Astrofisica 
de Canarias (IAC) (https://goto-observatory.org 
<https://goto-observatory.org/>)