[vsnet-grb-info 16229] GRB 150301A: Swift detection of a short burst or new SGR

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17510
SUBJECT: GRB 150301A: Swift detection of a short burst or new SGR
DATE:    15/03/01 01:26:16 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), M. De Pasquale (INAF-IASFPA),
L. M. Z. Hagen (PSU), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 01:04:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source which could be a short GRB or a new SGR (trigger=632995). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 244.286, -48.697, which is 
   RA(J2000)  =  16h 17m 09s
   Dec(J2000) = -48d 41' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
structure with a duration of about 0.1 sec.  The peak count rate
was ~8500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 01:05:26.6 UT, 58.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 244.3047, -48.7131 which is equivalent to:
   RA(J2000)  = +16h 17m 13.13s
   Dec(J2000) = -48d 42' 47.2"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 73 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 113 seconds with the White filter
starting 65 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 coverage of the XRT error circle by the 8'x8' region
for the list of sources generated on-board is uncertain because the large
number of sources filled the available telemetry. The list of sources is
typically complete to about 18 mag. No correction has been made for the large,
but uncertain extinction expected. 

We note that this source is near the Galactic Plane and Bulge 
(lat = 1.36, lon = 334) and so this might be a previously-unknown
Soft Gamma Repeater. The burst has significant emission above 100 keV,
which is unusual, but not unprecedented, for an SGR.  Further analysis
of the full data set and continued observation is required to
determine the nature of this source. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)