[vsnet-grb-info 20436] GRB 180325A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22532
SUBJECT: GRB 180325A: Swift detection of a burst with an optical counterpart
DATE:    18/03/25 02:04:12 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

E. Troja (NASA/GSFC/UMCP), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC),
J. A. Kennea (PSU), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU)
and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 01:53:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180325A (trigger=817564).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 157.432, +24.439 which is 
   RA(J2000) = 10h 29m 44s
   Dec(J2000) = +24d 26' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a ~10 second
FRED peak (~3000 counts/s) which is followed by brighter peak
which reaches a maximum of ~11000 counts/sec (15-350 keV), at ~81
after the trigger, for a total duration of ~120 seconds. 

The XRT began observing the field at 01:54:16.2 UT, 73.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 157.4281, 24.4627 which is equivalent to:
   RA(J2000)  = +10h 29m 42.74s
   Dec(J2000) = +24d 27' 45.7"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 86 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 150 seconds with the White filter
starting 82 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	10:29:42.59 = 157.42746
  DEC(J2000) = +24:27:48.5  =  24.46347
with a 90%-confidence error radius of about 0.76 arc sec. This position is 3.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.45 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

We note that the second peak seen by BAT occurs during the 
early UVOT and XRT images, so these sample the prompt GRB
emission. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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.)