[vsnet-grb-info 20896] GRB 180721A: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22994
SUBJECT: GRB 180721A: Swift detection of a burst
DATE:    18/07/21 11:20:41 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

B. Sbarufatti (PSU), F. E. Marshall (NASA/GSFC) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 11:06:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180721A (trigger=849018).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 347.695, +4.880 which is 
   RA(J2000) = 23h 10m 47s
   Dec(J2000) = +04d 52' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 60 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 11:07:45.3 UT, 56.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
347.7100, 4.8553 which is equivalent to:
   RA(J2000)  = 23h 10m 50.41s
   Dec(J2000) = +04d 51' 19.1"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 103 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
http://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 (6.47 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.7
(+3.22/-2.74) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.38e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
119 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
0.07. 

Burst Advocate for this burst is B. Sbarufatti (bxs60 AT psu.edu). 
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/too.html.)