[vsnet-grb-info 17770] GRB 160321A: Swift detection of a probable burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19211
SUBJECT: GRB 160321A: Swift detection of a probable burst
DATE:    16/03/21 16:11:48 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC),
A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), S. B. Cenko (GSFC),
P. D'Avanzo (INAF-OAB), P.A. Evans (U Leicester), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
L. M. McCauley (PSU), A. Melandri (INAF-OAB), B. Mingo (U Leicester),
K. L. Page (U Leicester), T. G. R. Roegiers (PSU), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 15:55:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160321A (trigger=680017).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 99.414, +5.715, which is 
   RA(J2000)  =  06h 37m 39s
   Dec(J2000) = +05d 42' 53"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a few weak peaks
with a total duration of about 35 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 15:57:18.0 UT, 110.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 99.4192,
5.7474 which is equivalent to:
   RA(J2000)  = 06h 37m 40.60s
   Dec(J2000) = +05d 44' 50.8"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 118 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 consistent with the Galactic value of 7.16
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 114 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 large, but
uncertain extinction expected. 

Due to the proximity of the source to the Galactic plane
(lat = -0.43 degrees) we cannot determine whether this is a
Galactic source or a GRB, based on the immediately available data. 

Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 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/too.html.)