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GCN Circular 30523

Subject
GRB 210726A: Swift detection of a burst
Date
2021-07-26T19:39:05Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
M. G. Bernardini (INAF-OAB), M. A. Baer (PSU), P. D'Avanzo (INAF-OAB),
J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
S. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), M. J. Moss (GWU), K. L. Page (U Leicester),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 19:19:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210726A (trigger=1061687).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 193.342, +19.185 which is 
   RA(J2000) = 12h 53m 22s
   Dec(J2000) = +19d 11' 05"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short spike with 
a tail with a total duration of about ~3 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 
This may be a short GRB. 

The XRT began observing the field at 19:20:03.6 UT, 60.0 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 193.2909,
19.1898 which is equivalent to:
   RA(J2000)  = 12h 53m 09.81s
   Dec(J2000) = +19d 11' 23.2"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 174 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.73 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.4
(+2.93/-2.51) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
276 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 18.0 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.020. 

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.inaf.it). 
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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