[vsnet-grb-info 9351] Trigger 424161: Swift/BAT trigger on XTE J1946+274/GRO J1944+26

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10834
SUBJECT: Trigger 424161: Swift/BAT trigger on XTE J1946+274/GRO J1944+26
DATE:    10/06/08 12:13:11 GMT
FROM:    David Burrows at PSU/Swift  <dxb15 at psu.edu>

M. De Pasquale (UCL-MSSL), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), P. A. Curran (UCL-MSSL), V. D'Elia (ASDC),
P. A. Evans (U Leicester), J. M. Gelbord (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), O. M. Littlejohns (U Leicester),
V. Mangano (INAF-IASFPA), J. Mao (INAF-OAB), R. Margutti (INAF-OAB),
S. R. Oates (UCL-MSSL), J. P. Osborne (U Leicester),
C. Pagani (U Leicester), K. L. Page (U Leicester),
P. Romano (INAF-IASFPA), A. Rowlinson (U Leicester),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC),
R. L. C. Starling (U Leicester), T. N. Ukwatta (GSFC/GWU) and
L. Vetere (PSU) report on behalf of the Swift Team:

At 11:20:42 UT, the Swift Burst Alert Telescope (BAT) triggered on the cataloged
source XTE J1946+274 (trigger=424161).  Swift slewed to the source as soon as
the BAT detected it as an image trigger.  The BAT on-board calculated location is 
RA, Dec 296.430, +27.366 which is 
   RA(J2000) = 19h 45m 43s
   Dec(J2000) = +27d 21' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). 

The XRT began observing the field at 11:51:28.2 UT, 1846.1 seconds
after the BAT trigger. Using promptly downlinked data we find a bright
X-ray source located at RA, Dec 296.41360, 27.36422 which is equivalent
to:
   RA(J2000)  = 19h 45m 39.26s
   Dec(J2000) = +27d 21' 51.2"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 53 arcseconds from the BAT onboard position, within the
BAT error circle. This is 4.5 arcsec from the position of XTE J1946+274
(2MASS J19453935+2721555).  The XRT 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 (9.43e+21
cm^-2, Kalberla et al. 2005), with an excess column of 7.8
(+9.36/-7.22) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.09e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 1854 seconds after the BAT trigger. No credible optical transient
has been found in the initial data products. The 2.7'x2.7' sub-image 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
sub-image. No correction has been made for the large, but uncertain
extinction expected.