[vsnet-grb-info 17860] GRB 160409A found in Swift BAT ground analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19304
SUBJECT: GRB 160409A found in Swift BAT ground analysis
DATE:    16/04/12 19:20:11 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

GRB 160409A found in Swift BAT ground analysis

P.A. Evans (U. Leicester),  J.P. Osborne (U Leicester), 
K. L. Page (U Leicester), and  D.M. Palmer (LANL)
report on behalf of the Swift Team

At 05:54:14 UT on 2016-04-09 BAT triggered and located
GRB 160409A (trigger #682162).  Swift did not immediately 
slew to this source because its location approximately
matched a known source (4U 1916-053) in its on-board catalog.  
The BAT on-board calculated location is 
RA, dec 289.705,-5.390 which is 
RA(J2000) = 19h 18m 49s
Dec(J2000) = -5d 23' 24"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
peak structure with a duration of about 30 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~2 sec after the trigger.

The catalogued source, 4U 1916-053, is identified by Simbad as being
the LXB V* V1405 Aql.  However, this object is 9 arcminutes from the 
BAT position, and well outside the error circle.  BAT has detected a burst
from that source (1.1 arcmin offset from the V1405 Aql location)
in April 2006.  During the 6-minute accumulation that overlaps the
trigger, SAX J2103.5+4545 is detected (at lower significance and 
correspondingly larger position uncertainty) with a 3.3 arcmin offset.
Therefore we can confidently state that this trigger did not come
from the location of V1405 Aql and is due to a different source.

A Swift Target of Opportunity was requested and XRT began
observing the field at 00:34:58 UT on 2016-04-12, which is
2.8 days after the BAT trigger. No unknown sources were 
detected in the 3 kiloseconds of data, although two known 
sources and possible scattered X-rays from V1405 Aql were seen.
No new sources were detected in UVOT down to a limiting
magnitude of 21.3 in white.

Qualitatively, about half of long GRBs would be visible in a 
3 ks XRT observation at that delay after the trigger, so the 
non-detection of an afterglow does not rule out a GRB.