[vsnet-grb-info 27929] GRB 210517A: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30032
SUBJECT: GRB 210517A: Swift detection of a burst
DATE:    21/05/17 05:52:10 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

V. D'Elia (SSDC & INAF-OAR), E. Ambrosi (INAF-IASFPA),
A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU), H. A. Krimm (NSF),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 05:28:01 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210517A (trigger=1048783).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 358.174, -39.105 which is 
   RA(J2000) = 23h 52m 42s
   Dec(J2000) = -39d 06' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT lightcurve shows a peak at least
2 seconds long and a maximum count rate of ~2200 counts/s (15-350 keV). 

The XRT began observing the field at 05:29:08.6 UT, 67.0 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 358.2245,
-39.1020 which is equivalent to:
   RA(J2000)  = 23h 52m 53.88s
   Dec(J2000) = -39d 06' 07.2"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 141 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 (1.30 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.2
(+2.58/-2.26) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 69 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 0.011. 

Burst Advocate for this burst is V. D'Elia (valerio.delia AT ssdc.asi.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/)