GCN Circular 3987

A. Falcone (PSU), A. Beardmore (U. Leicester), D. Burrows (PSU),
J. Cummings (GSFC/NRC), D. Fox (PSU), N. Gehrels (GSFC), 
S. T. Holland (GSFC/UMBC), J. Kennea (PSU), H. Krimm (GSFC/USRA),
C. Pagani (PSU), D. Palmer (LANL), A. Parsons (GSFC), E. Rol (U. Leicester),
P. Roming (PSU), P. Shady (MSSL),
on behalf of the Swift team:

At 21:23:04 UT, Swift-BAT triggered and located GRB050915B (trigger=155284).
The spacecraft slewed immediately.  The BAT on-board calculated location
is RA,Dec 219.096d,-67.405d  {14h 36m 23s, -67d 24' 16"} (J2000), with an 
uncertainty of 3 arcmin (radius, 90% containment, stat+sys).  The BAT light 
curve showed two broad bright peaks from T-10 to T+40 sec.  The peak count 
rate was ~2800 counts/sec (15-350 keV), at T-2 sec.

XRT began observing at 21:25:20 UT, 136 seconds after the BAT trigger.
XRT was not able to centroid on any point source, but the downlinked
lightcurve shows that a bright, fading X-ray source was present in the
XRT field of view.

The UVOT began observing at 21:25:18 UT, 134 seconds after the BAT trigger.
No new source, with respect to the DSS, is detected in the initial 100 sec 
V-band image.  The 5-sigma limiting magnitude, in a circular aperture of
radius 6.0 arcseconds is V_lim = 17.7 mag.  This magnitude is based on the 
preliminary V-band zero-point, measured in orbit and will require refinement 
with further calibration.  It has not been corrected for extinction.  The 
estimated Galactic extinction in this direction is A_V = 1.29.  The UVOT 
image covers 25% of the 3 arcminute BAT error circle. The field is crowded.

We are currently in the portion of the orbits where the spacecraft does
not pass over the Malindi downlink station. Therefore, it will be ~3
hours before we have access to the full data set for the refined analyses.