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GCN Circular 18373

Subject
GRB 151001A: Swift detection of a burst with an optical counterpart
Date
2015-10-01T16:38:56Z (8 years ago)
From
Jamie A. Kennea at PSU/Swift-XRT <kennea@swift.psu.edu>
M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
S. L. Gibson (U Leicester), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
C. Pagani (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 15:04:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151001A (trigger=657286).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 233.731, +10.971 which is 
   RA(J2000) = 15h 34m 55s
   Dec(J2000) = +10d 58' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a couple overlapping peaks
with a duration of about 100 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 15:05:26.9 UT, 64.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 233.7289, 10.9669 which
is equivalent to:
   RA(J2000)  = 15h 34m 54.94s
   Dec(J2000) = +10d 58' 00.8"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 16 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 (3.53 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.3
(+1.39/-1.27) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 67 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	15:34:54.90 = 233.72874
  DEC(J2000) = +10:58:02.1  =  10.96724
with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.94 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 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.)
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