{
  "bibcode": "2021GCN.29262....1S",
  "body": "S. J. Smartt, K. W. Smith (QUB), S. Srivastav,, D. R.  Young,\nM. Fulton, (QUB) L. Denneau, A. Heinze, J.  Tonry, H. Weiland (IfA,\nUniv. Hawaii), A. Rest (STScI), B. Stalder (LSST), C. Stubbs\n(Harvard), O. McBrien, J. Gillanders, L. Shingles (QUB), \nT.-W. Chen (MPE)\n\nWe report serendipitous observations of GRB210104A (Troja et al. GCN\n29233) by ATLAS, the twin telescope system on Haleakala and Mauna\nLoa that surveys the whole northern sky every 2 days (see Tonry et\nal. 2018, PASP, 130f4505).  Optical transients are automatically\ndetected on the difference images and discoveries (Smith et al. 2020,\nPASP, 132h5002) are made public through the IAU Transient Name Server.\n\nDuring the course of its survey, ATLAS happened to be observing the\nfield of GRB210104A starting 1min 47sec after the Swift BAT trigger at\n2021-01-04 11:26:59 UT (Troja et al. GCN 29233). This was a fortunate\ncoincidence in normal survey mode and not a triggered pointing.\n\nWe detect the bright UVOT afterglow as initially reported in (Troja et\nal. GCN 29233), and appear to have the caught it early and bright at\no = 13.9 along with rapid fading (see also Xin et al, GCN29235, Hu et\nal. 29236, Hosokawa et al. GCN 29237, Kim et al. GCN 29238, Lipunov et\nal. 29239),\n\nIt was detected at a position : \nRA = 06:55:05.37 (103.77240) DEC = +64:40:34.2 (+64.67618) \n              \nThe expoures were 30sec long in the o-band (an r+i composite),\nwith start time and magnitudes as follows : \n\n###MJD              UTC                                         m      dm  \n59218.478319  2021-01-04 11:28:46.762 UTC 13.94  0.01\n59218.481544  2021-01-04 11:33:25.402 UTC 15.56  0.01\n59218.487080  2021-01-04 11:41:23.712 UTC 16.43  0.02\n59218.497226  2021-01-04 11:56:00.326 UTC 16.96  0.03\n59218.670448  2021-01-04 16:05:26.707 UTC >18.4  (3 sigma limit)",
  "circularId": 29262,
  "createdOn": 1609868871000,
  "email": "s.smartt@qub.ac.uk",
  "subject": "GRB 210104A: early ATLAS detections of the bright afterglow",
  "submitter": "Stephen Smartt at Queen's U/Belfast  <s.smartt@qub.ac.uk>",
  "eventId": "GRB 210104A"
}