[vsnet-grb-info 5331] GRB 071028B Swift-BAT detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7019
SUBJECT: GRB 071028B Swift-BAT detection of a burst
DATE:    07/10/30 13:19:51 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <jayc at milkyway.gsfc.nasa.gov>

J. R. Cummings (CRESST/NASA/GSFC) and J. Greiner (MPE) for the Swift team

At 02:43:46 UT on October 28, the Swift Burst Alert Telescope (BAT)
triggered on GRB 071028B (trigger #295492). The burst was on the edge
of the field of view (4.1% coded) and no source was found automatically
onboard. A source was found during later ground analysis at a location
RA, Dec 354.163, -31.630 which is
   RA(J2000)  =  23h 36m 39s
   Dec(J2000) = -31d 37' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows two FRED peaks,
the first at T+1 sec about 4 seconds long, the second at T+48 sec about
9 seconds long. The peak count rate was about 1500 counts/sec
(15-350 keV), at ~1 sec after the trigger.

Because the burst was detected on the ground, there are no prompt
XRT or UVOT data. A Swift TOO has been scheduled for October 30,
at approximately 1400 UT.

There have been several ground-detected Swift-BAT bursts lately:
GRB 071006 (GCN circ #6858), GRB 071010C (GCN circ #6906) and
this one. We have not changed our procedures substantially, and
we do not believe that we have missed many such bursts in the past.
It apparently is merely an odd statistical fluctuation of rare
events: Two weak bursts on the edge of the BAT field of view and
another immediately before a preplanned slew maneuver.

Five such bursts have been noted before, one occurring just before
a preplanned slew, GRB 051012  (GCN circ #4093), one combining both
factors, GRB 070227 (GCN circ #6156), and the other three being very
weak/short, GRB 051114 (GCN circ #4272), GRB 060505 (GCN circ #5076),
and GRB 070406 (GCN circ #6247). A fifth nearly similar case was a
very long and weak burst, GRB 060123 (GCN circ #4608), producing a
significant detection onboard though automatic long-image triggers
were not enabled at that time.