[vsnet-grb-info 4534] Trigger 276045: Swift detection of a possible burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6299
SUBJECT: Trigger 276045: Swift detection of a possible burst
DATE:    07/04/18 11:50:45 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

C. Guidorzi (Univ Bicocca&INAF-OAB), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), N. Gehrels (NASA/GSFC),
S. T. Holland (CRESST/GSFC/USRA), J. A. Kennea (PSU),
H. A. Krimm (GSFC/USRA), W. B. Landsman (NASA/GSFC),
V. Mangano (INAF-IASFPA), C. B. Markwardt (GSFC/UMD),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
J. P. Osborne (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Perri (ASDC),
P. Romano (Univ. Bicocca & INAF-OAB), B. Sbarufatti (INAF-IASFPA),
P. Schady (MSSL-UCL), M. Stamatikos (NASA/ORAU),
G. Tagliaferri (INAF-OAB), E. Troja (INAF-IASFPA),
D. E. Vanden Berk (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf
of the Swift Team:

At 11:21:01 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a marginal-significance image peak that may be GRB 070418
(trigger=276045).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 228.504, +51.226 which is 
   RA(J2000) = 15h 14m 01s
   Dec(J2000) = +51d 13' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single
0.064 second timebin with an excess of ~100 counts above background,
primarily in the 100-350 keV energy bin.  The very short duration 
and confinement to the highest energy channel indicate that this
is very likely to be a cosmic ray shower, although a short, weak,
very hard GRB cannot be eliminated at this time.  A final 
determination of the cause of this event will 
require data from the next Malindi pass, which will be available
around 1300 UT. 

The XRT began taking data at 11:22:22 UT, 81 seconds after the BAT
trigger.  The XRT on-board centroid algorithm did not find a source in the
image and no prompt position is available.  Down-linked
data (for a total exposure of 1140s) show no sources in the XRT field. 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 85 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 18.5 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for extinction of E(B-V)=0.02.