TITLE:   GCN CIRCULAR
NUMBER:  28161
SUBJECT: GRB 200723A: ATLAS observations of GOTO candidate AT2020qer 
DATE:    20/07/24 18:21:59 GMT
FROM:    Ken Smith at Queen's University Belfast  <k.w.smith@qub.ac.uk>

K. W. Smith, S. Srivastav, O. McBrien, S. J. Smartt, J. Gillanders,
P. Clark, M. Fulton, D. O'Neill, D. R. Young, M. McCollum (QUB),
A. Townsend (University of Manchester, QUB), T.-W. Chen (Stockholm),
J. Anderson (ESO), L. Denneau, A. Heinze, J. Tonry, H. Weiland (IfA,
University of Hawaii), B. Stalder (LSST), A. Rest (STScI), D. E. Wright
(University of Minnesota)

The transient AT2020qer reported by Gompertz et al (GCN 28159) as a
possible counterpart to GRB 200723A (Fermi GBM; GCN 28155), was detected
independently by ATLAS, the twin telescope system on Haleakala and Mauna
Loa as part of normal sky operations (Tonry et al. 2018, PASP, 130f4505,
Smith et al. 2020, PASP, 132f1014) on 2020-07-24.32 (MJD 59054.32).

Forced photometry at that position shows clear, rising activity before
the GRB event.

ATLAS forced photometry:
MJD          dt   m      dm    filter
59048.36523 -5.33 19.50  0.15  c
59048.36843 -5.32 20.13  0.24  c
59048.37442 -5.32 19.36  0.14  c
59048.40300 -5.29 19.47  0.15  c
59050.33364 -3.36 18.89  0.09  o
59050.33733 -3.35 19.10  0.11  o
59050.34287 -3.35 18.95  0.10  o
59050.35486 -3.34 19.01  0.10  o
59054.31747  0.63 18.18  0.06  o
59054.32069  0.63 18.20  0.05  o
59054.32670  0.64 18.34  0.06  o
59054.33871  0.65 18.27  0.06  o

where dt is the time difference between ATLAS MJD and the GRB trigger.

We therefore conclude that AT2020qer is unrelated to GRB 200723A and is
probably a young supernova candidate.