[vsnet-grb-info 26371] GRB 200922A: Swift detection of a burst with possible optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28471
SUBJECT: GRB 200922A: Swift detection of a burst with possible optical counterpart
DATE:    20/09/22 12:26:24 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

M. J. Moss (GWU), E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (PSU) and M. H. Siegel (PSU) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 12:06:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200922A (trigger=997024).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 296.915, -55.204 which is 
   RA(J2000) = 19h 47m 40s
   Dec(J2000) = -55d 12' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 10 sec.  The peak count rate
was ~10000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:08:02.2 UT, 75.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 296.95425, -55.20245 which
is equivalent to:
   RA(J2000)  = 19h 47m 49.02s
   Dec(J2000) = -55d 12' 08.8"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 80 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 consistent with the Galactic value of 4.71
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 8.21e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 85 seconds after the BAT trigger. The 2.7'x2.7'
sub-image covers 100% of  the XRT error circle. There is a 14.4 mag
source at (296.95361,-55.20447)  8.3 arc seconds from the center of
the XRT error region. There is a  much weaker source at the same
position in the DSS. No  correction has been made for the expected
extinction corresponding to E(B-V) of  0.05. 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 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/)