[vsnet-grb-info 16384] GRB 150403A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17665
SUBJECT: GRB 150403A: Swift detection of a burst with an optical counterpart
DATE:    15/04/03 22:17:05 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), V. D'Elia (ASDC),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 21:54:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150403A (trigger=637044).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 311.505, -62.702, which is 
   RA(J2000)  =  20h 46m 01s
   Dec(J2000) = -62d 42' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows two overlapping peaks
with a total duration of about 60 sec.  The peak count rate
was ~19,000 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 21:55:31.5 UT, 74.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 311.5019, -62.7118 which is equivalent to:
   RA(J2000)  = +20h 46m 0.46s
   Dec(J2000) = -62d 42' 42.5"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 35 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. 

The initial flux in the 2.5 s image was 6.34e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 84 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	20:46:01.14 = 311.50476
  DEC(J2000) = -62:42:41.0  = -62.71138
with a 90%-confidence error radius of about 0.61 arc sec. This position is 5.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.85 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.05. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)