GCN Circular 19343

H. A. Krimm (CRESST/GSFC/USRA), A. D'Ai (INAF-IASFPA),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL),
J. L. Racusin (NASA/GSFC) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 23:26:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160425A (trigger=684098).  Swift slewed to the burst
after a short delay due to an observing constraint. 
The BAT on-board calculated location is 
RA, Dec 280.313, -54.359 which is 
   RA(J2000) = 18h 41m 15s
   Dec(J2000) = -54d 21' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of about 2 sec, followed by a 
50 second-long peak at T+250s.  There may also be a
precursor peak of ~100 s duration at T-380s. The peak count rate
was ~6000 counts/sec (15-350 keV), at ~0.2 sec after the trigger. 

The XRT began observing the field at 23:29:34.7 UT, 203.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 280.3274,
-54.3601 which is equivalent to:
   RA(J2000)  = 18h 41m 18.56s
   Dec(J2000) = -54d 21' 36.2"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 30 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 in excess of the Galactic value (7.74 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.5
(+2.72/-2.31) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 208 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:41:18.57 = 280.32738
  DEC(J2000) = -54:21:36.1  = -54.36004
with a 90%-confidence error radius of about 0.68 arc sec. This position is 2.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.91 with a 1-sigma error of about  0.17. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.06. 

It is extremely unusual for a short GRB to have a long pulse
a few minutes later with much more fluence. 
It is more common for a long GRB to have a short precursor pulse. 
Further characterization of this event will require the full
downlinked data set. 

Burst Advocate for this burst is H. A. Krimm (hans.krimm 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.)