{
  "bibcode": "2023GCN.33278....1S",
  "body": "S. J. Smartt (Oxford), K. W. Smith, S. Srivastav, D. R. Young,,\nM. Fulton, M. McCollum, T. Moore, J. Weston (Queen's University\nBelfast), L. Shingles (GSI/QUB), L. Rhodes (Oxford), L. Denneau, J.\nTonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii),\nN. Erasmus, W. Koorts (South African Astronomical Observatory), A.\nJordan, V. Suc (UAI, Obstech), A. Rest (STScI), T.-W. Chen (TUM/MPA),\nM. Nicholl (Birmingham), C. Stubbs (Harvard), J. Sommer (LMU/QUB)\n\nWe report early optical observations of the afterglow of the Maxi/GSC\ndiscovered GRB 230204B (Serino et al. GCN 33265). The optical\nafterglow was discovered by Kumar et al. (GCN 29233) at r=15.50 +/-\n0.04 with a decline rate of 1.2 mag per hr. Kumar et al. reported this\noptical transient discovery as AT2023bic to the Transient Name Server.\n\nWe observed the field with the ATLAS system in normal survey mode.\nATLAS is a quadruple 0.5m telescope system with two units in Hawaii,\nand one each in Chile and South Africa (see Tonry et al. 2018,\nPASP,130:064505). Our transient science server (Smith et al. 2020,\nPASP, 132:085002) independently detected AT2023bic on the ATLAS\n(Sutherland) images at coordinates RA = 13:10:34.93, Declination =\n-21:43:05.1 (J2000).\n\nForced photometry at those coordinates results in the following AB\nmagnitudes in the o-band filter (a broad r+i composite filter) :\n\nMJD                   Date                             mag   dmag\n59980.05525005 2023-02-05 01:19:33 17.05 0.04\n59980.05708335 2023-02-05 01:22:12 17.18 0.05\n59980.07028305 2023-02-05 01:41:12 17.33 0.05\n59980.07343935 2023-02-05 01:45:45 17.32 0.05\n\nWe measure a decline rate of 0.6 mag per hr, approximately 2hrs after\nthe Kumar et al. measurement. The decline may be slowing.\n\nWe note that AT2023bic is located 108.60\" N, 10.90\" W from the nearby\ngalaxy ESO 576- G 003, at z = 0.009873 (approximately 40Mpc). This\ngalaxy sits within the PGC1 0045721 galaxy group (NED and Kourkchi &\nTully 2017, ApJ 843, 16). While this is likely a line of sight\ncoincidence, a spectroscopic redshift of AT2023bic is needed. If it\nwere at a distance of 40Mpc, it would be 22kpc projected offset from\nESO 576- G 003. There is no source in the Pan-STARRS 3Pi images at\nthis positon (Chambers et al. arXiv:1612.05560).",
  "circularId": 33278,
  "createdOn": 1675639636000,
  "email": "s.smartt@qub.ac.uk",
  "subject": "GRB 230204B: ATLAS detection of the optical afterglow and a projected location in a nearby galaxy group",
  "submitter": "Stephen Smartt at Queen's U/Belfast  <s.smartt@qub.ac.uk>",
  "eventId": "GRB 230204B"
}