{
  "bibcode": "2021GCN.29243....1L",
  "body": "S. Laha (GSFC/UMBC/CRESST), M. G. Bernardini (INAF-OAB),\nA. A. Breeveld (UCL-MSSL), P. D'Avanzo (INAF-OAB), C. Gronwall (PSU),\nJ.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU),\nA. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),\nA. Melandri (INAF-OAB), M. J. Moss (GWU), K. L. Page (U Leicester),\nD. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and M. H. Siegel (PSU)\nreport on behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 21:10:10 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 210104B (trigger=1015919).  Swift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 53.601, +37.931 which is \n   RA(J2000) = 03h 34m 24s\n   Dec(J2000) = +37d 55' 51\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a double-peaked\nstructure with a duration of about 30 sec.  The peak count rate\nwas ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. \n\nThe XRT began observing the field at 21:11:31.2 UT, 80.5 seconds after\nthe BAT trigger. Using promptly downlinked data we find an uncatalogued\nX-ray source with an enhanced position: RA, Dec 53.5539, 37.9302 which\nis equivalent to:\n   RA(J2000)  = 03h 34m 12.93s\n   Dec(J2000) = +37d 55' 48.7\"\nwith an uncertainty of 2.5 arcseconds (radius, 90% containment). This\nlocation is 133 arcseconds from the BAT onboard position, within the\nBAT error circle. This position may be improved as more data are\nreceived; the latest position is available at\nhttps://www.swift.ac.uk/sper.  We cannot determine whether the source\nis fading at the present time. \n\nA power-law fit to a spectrum formed from promptly downlinked event\ndata gives a column density in excess of the Galactic value (2.67 x\n10^21 cm^-2, Willingale et al. 2013), with an excess column of 1.6\n(+1.04/-0.80) x 10^22 cm^-2 (90% confidence). \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 83 seconds after the BAT trigger. No credible afterglow candidate has\nbeen found in the initial data products. The 2.7'x2.7' sub-image covers none of\nthe XRT error circle. The 8'x8' region for the list of sources generated\non-board covers 100% of the XRT error circle. The list of sources is typically\ncomplete to about 18 mag. No correction has been made for the expected\nextinction corresponding to E(B-V) of 0.79. \n\nBurst Advocate for this burst is S. Laha (sib.laha AT gmail.com). \nPlease contact the BA by email if you require additional information\nregarding Swift followup of this burst. In extremely urgent cases, after\ntrying the Burst Advocate, you can contact the Swift PI by phone (see\nSwift TOO web site for information: http://www.swift.psu.edu/)",
  "circularId": 29243,
  "createdOn": 1609795485000,
  "email": "palmer@lanl.gov",
  "subject": "GRB 210104B: Swift detection of a burst",
  "submitter": "David Palmer at LANL  <palmer@lanl.gov>",
  "eventId": "GRB 210104B"
}