[vsnet-grb-info 16234] GRB 150301B: Swift detection of a burst with an optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17515
SUBJECT: GRB 150301B: Swift detection of a burst with an optical afterglow
DATE:    15/03/01 19:50:01 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. Y. Lien (GSFC/UMBC), D. N. Burrows (PSU), M. M. Chester (PSU),
J. R. Cummings (NASA/UMBC), M. De Pasquale (INAF-IASFPA),
A. Maselli (INAF-IASFPA), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 19:38:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150301B (trigger=633180).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 89.157, -57.977 which is 
   RA(J2000) = 05h 56m 38s
   Dec(J2000) = -57d 58' 37"
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 20 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 19:39:26.4 UT, 82.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 89.16636, -57.97051 which is equivalent
to:
   RA(J2000)  = 05h 56m 39.93s
   Dec(J2000) = -57d 58' 13.8"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 29 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 consistent with the Galactic value of 6.16
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:56:39.86 =  89.16608
  DEC(J2000) = -57:58:10.4  = -57.96956
with a 90%-confidence error radius of about 0.61 arc sec. This position is 31.9
arc sec. from the center of the BAT error circle. The estimated magnitude is
16.22 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.08. 

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.)