TITLE:   GCN CIRCULAR
NUMBER:  11159
SUBJECT: GRB 100901A: Swift detection of a burst with an optical afterglow
DATE:    10/09/01 14:00:55 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. Immler (CRESST/GSFC/UMD), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
S. Campana (INAF-OAB), V. D'Elia (ASDC), P. A. Evans (U Leicester),
J. M. Gelbord (PSU), O. Godet (U Leicester), C. Gronwall (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
E. A. Hoversten (PSU), O. M. Littlejohns (U Leicester),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
J. P. Osborne (U Leicester), C. Pagani (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. A. Pritchard (PSU),
A. Rowlinson (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 13:34:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100901A (trigger=433065).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 27.245, +22.744, which is 
   RA(J2000)  =  01h 48m 59s
   Dec(J2000) = +22d 44' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single symmetric
peak structure with a duration of about 10 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~1 sec after T_zero. 

The XRT began observing the field at 13:36:51.0 UT, 160.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 27.2642, 22.7584 which
is equivalent to:
   RA(J2000)  = 01h 49m 3.41s
   Dec(J2000) = +22d 45' 30.1"
with an uncertainty of 3.0 arcseconds (radius, 90% containment). This
location is 82 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
7.08e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 147 seconds after the BAT trigger. There is a
candidate afterglow in  the rapidly available 2.7'x2.7' sub-image at 
  RA(J2000)  =	01:49:03.42 =  27.26424 
  DEC(J2000) = +22:45:30.8  =  22.75856  with a 90%-confidence error
radius of about 0.81 arc sec. This position is 0.6  arc sec. from the
center of the XRT-Enhanced error circle. The estimated magnitude is
19.67 with a 1-sigma error of about  0.18. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.10. 

Burst Advocate for this burst is S. Immler (immler AT milkyway.gsfc.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.)