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GCN Circular 33278

Subject
GRB 230204B: ATLAS detection of the optical afterglow and a projected location in a nearby galaxy group
Date
2023-02-05T23:27:16Z (a year ago)
From
Stephen Smartt at Queen's U/Belfast <s.smartt@qub.ac.uk>
S. J. Smartt (Oxford), K. W. Smith, S. Srivastav, D. R. Young,,
M. Fulton, M. McCollum, T. Moore, J. Weston (Queen's University
Belfast), L. Shingles (GSI/QUB), L. Rhodes (Oxford), L. Denneau, J.
Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii),
N. Erasmus, W. Koorts (South African Astronomical Observatory), A.
Jordan, V. Suc (UAI, Obstech), A. Rest (STScI), T.-W. Chen (TUM/MPA),
M. Nicholl (Birmingham), C. Stubbs (Harvard), J. Sommer (LMU/QUB)

We report early optical observations of the afterglow of the Maxi/GSC
discovered GRB 230204B (Serino et al. GCN 33265). The optical
afterglow was discovered by Kumar et al. (GCN 29233) at r=15.50 +/-
0.04 with a decline rate of 1.2 mag per hr. Kumar et al. reported this
optical transient discovery as AT2023bic to the Transient Name Server.

We observed the field with the ATLAS system in normal survey mode.
ATLAS is a quadruple 0.5m telescope system with two units in Hawaii,
and one each in Chile and South Africa (see Tonry et al. 2018,
PASP,130:064505). Our transient science server (Smith et al. 2020,
PASP, 132:085002) independently detected AT2023bic on the ATLAS
(Sutherland) images at coordinates RA = 13:10:34.93, Declination =
-21:43:05.1 (J2000).

Forced photometry at those coordinates results in the following AB
magnitudes in the o-band filter (a broad r+i composite filter) :

MJD                   Date                             mag   dmag
59980.05525005 2023-02-05 01:19:33 17.05 0.04
59980.05708335 2023-02-05 01:22:12 17.18 0.05
59980.07028305 2023-02-05 01:41:12 17.33 0.05
59980.07343935 2023-02-05 01:45:45 17.32 0.05

We measure a decline rate of 0.6 mag per hr, approximately 2hrs after
the Kumar et al. measurement. The decline may be slowing.

We note that AT2023bic is located 108.60" N, 10.90" W from the nearby
galaxy ESO 576- G 003, at z = 0.009873 (approximately 40Mpc). This
galaxy sits within the PGC1 0045721 galaxy group (NED and Kourkchi &
Tully 2017, ApJ 843, 16). While this is likely a line of sight
coincidence, a spectroscopic redshift of AT2023bic is needed. If it
were at a distance of 40Mpc, it would be 22kpc projected offset from
ESO 576- G 003. There is no source in the Pan-STARRS 3Pi images at
this positon (Chambers et al. arXiv:1612.05560).
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