GCN Circular 17261

A. Vargas (PSU), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (NASA/UMBC), H. A. Krimm (CRESST/GSFC/USRA),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 06:28:53 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150101A (trigger=623158).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 312.631, +36.728 which is 
   RA(J2000) = 20h 50m 31s
   Dec(J2000) = +36d 43' 39"
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 0.1 sec.  The peak count rate
was ~3700 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 06:29:54.4 UT, 60.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 312.6038, 36.7334 which
is equivalent to:
   RA(J2000)  = 20h 50m 24.91s
   Dec(J2000) = +36d 44' 00.3"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 80 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 4.62
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 65 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. 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 large, but uncertain
extinction  expected. 

Burst Advocate for this burst is A. Vargas (vargas.09242011.angie AT gmail.com). 
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.)