[vsnet-grb-info 27559] GRB 210318A: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29662
SUBJECT: GRB 210318A: Swift detection of a burst
DATE:    21/03/18 04:09:40 GMT
FROM:    Caryl Gronwall at PSU/Swift-UVOT  <caryl at swift.psu.edu>

E. Troja (NASA/GSFC/UMCP), C. Gronwall (PSU), J.D. Gropp (PSU),
N. J. Klingler (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 03:54:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210318A (trigger=1037339).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 33.930, -37.789 which is 
   RA(J2000) = 02h 15m 43s
   Dec(J2000) = -37d 47' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 30 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 03:56:35.2 UT, 123.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 33.9263, -37.7772 which
is equivalent to:
   RA(J2000)  = 02h 15m 42.32s
   Dec(J2000) = -37d 46' 38.1"
with an uncertainty of 3.2 arcseconds (radius, 90% containment). This
location is 43 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 1.88
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 126 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 19.6 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 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.016. 

Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov). 
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/)