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GCN Circular 30765

Subject
GRB 210905A: Swift detection of a burst
Date
2021-09-05T00:28:29Z (2 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
E. Sonbas (Adiyaman Univ.), E. Ambrosi (INAF-IASFPA),
A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR), C. Gronwall (PSU),
S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (U Tampa),
F. E. Marshall (NASA/GSFC) and D. M. Palmer (LANL) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 00:12:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210905A (trigger=1071993).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 309.062, -44.431 which is 
   RA(J2000) = 20h 36m 15s
   Dec(J2000) = -44d 25' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 30 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 00:14:13.0 UT, 91.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 309.04815, -44.43959 which
is equivalent to:
   RA(J2000)  = 20h 36m 11.56s
   Dec(J2000) = -44d 26' 22.5"
with an uncertainty of 7.0 arcseconds (radius, 90% containment). This
location is 47 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 1.23e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
156 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 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. 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
0.034. 

Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.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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