[vsnet-grb-info 19298] Trigger 745966: Swift detection of 4U1850-03

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20993
SUBJECT: Trigger 745966: Swift detection of 4U1850-03
DATE:    17/04/06 12:53:01 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), A. Cholden-Brown (PSU),
A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), A. Y. Lien (GSFC/UMBC),
B. Mingo (U Leicester), K. L. Page (U Leicester),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 12:29:01 UT, the Swift Burst Alert Telescope (BAT) triggered and
located 4U1850-03 (trigger=745966).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 282.038, -2.439, which is 
   RA(J2000)  =  18h 48m 09s
   Dec(J2000) = -02d 26' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a total duration of about 300 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV). 

The XRT began observing the field at 12:33:02.2 UT, 241.1 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. It is likely that the source was very bright and the instrument 
went directly to Windowed Timing mode, preventing us from getting a 
prompt position. 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 245 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 BAT error circle. The typical
3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the
list of sources generated  on-board covers 25% of the BATT error
circle. The list of sources is typically  complete to about 18 mag. No
correction has been made for the  large, but uncertain extinction
expected.