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GCN Circular 29243

Subject
GRB 210104B: Swift detection of a burst
Date
2021-01-04T21:24:45Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
S. Laha (GSFC/UMBC/CRESST), M. G. Bernardini (INAF-OAB),
A. A. Breeveld (UCL-MSSL), P. D'Avanzo (INAF-OAB), C. Gronwall (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), M. J. Moss (GWU), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 21:10:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210104B (trigger=1015919).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 53.601, +37.931 which is 
   RA(J2000) = 03h 34m 24s
   Dec(J2000) = +37d 55' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 21:11:31.2 UT, 80.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 53.5539, 37.9302 which
is equivalent to:
   RA(J2000)  = 03h 34m 12.93s
   Dec(J2000) = +37d 55' 48.7"
with an uncertainty of 2.5 arcseconds (radius, 90% containment). This
location is 133 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
https://www.swift.ac.uk/sper.  We cannot determine whether the source
is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.67 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 1.6
(+1.04/-0.80) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.79. 

Burst Advocate for this burst is S. Laha (sib.laha AT gmail.com). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
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