GCN Circular 13875

D. Grupe (PSU), B. N. Barlow (PSU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC),
E. A. Helder (PSU), S. T. Holland (STScI), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (NASA/GSFC),
F. E. Marshall (NASA/GSFC), C. J. Mountford (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
C. A. Swenson (PSU) report on behalf of the Swift Team:

At 19:23:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121017A (trigger=536172).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 288.816, -1.595 which is 
   RA(J2000)  =  19h 15m 16s
   Dec(J2000) = -01d 35' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows at least two peaks 
with a duration of about 10 sec (possibly longer).  The peak count rate 
was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 19:25:03.3 UT, 95.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 288.8302, -1.6042 which
is equivalent to:
   RA(J2000)  = 19h 15m 19.25s
   Dec(J2000) = -01d 36' 15.1"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 60 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 (2.22 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3
(+2.89/-2.40) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 1141 seconds after the BAT trigger. No credible afterglow candidate
has been found in the initial data products. The 2.7'x2.7' sub-image covers
100% of the XRT error circle. There is no new source down to a limit of
about 19th mag. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.44. 

XRT and UVOT detections were delayed due to entry into the SAA shortly
after the initial trigger. 

Burst Advocate for this burst is D. Grupe (grupe AT astro.psu.edu). 
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.)