GCN Circular 3991

D. Morris (PSU), D. Burrows (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC),
C. Gronwall (PSU), S. T. Holland (GSFC/UMBC), J. Kennea (PSU),
K. Page (U. Leicester), D. Palmer (LANL)
on behalf of the Swift team:

At 16:35:52 UT, Swift-BAT triggered and located GRB050916 (trigger=155408).
The spacecraft slewed immediately after processing the 120-second long
image.  The BAT on-board calculated location is RA,Dec 135.992d, -51.425d 
{09h 03m 58s, -51d 25' 31"} (J2000), with an uncertainty of 3 arcmin (radius,
90% containment, stat+sys).  The BAT raw light curve showed one very broad
peak from T+0 to T+180 sec, with a spike at about T+40 to T+45 sec.  The peak
count rate was ~500 counts/sec (15-350 keV), at T+40 sec.  However, Swift was
in the sunlit part of its orbit so the current heightened solar activity can
produce variation in the BAT raw detector count rates, and a trustworthy
light curve will require analysis of the full data set after the next ground
contact.

XRT began observing at 16:39:22 UT, 210 seconds after the BAT trigger.  The
count rate was too low for an on-board centroid. The spectrum is consistent
with a weak source. The XRT position will be available following the next 
ground station contact.

The UVOT began observing at 16:39:20 UT, 208 seconds after the BAT trigger.
No UVOT image was sent through TDRSS, however, the processed source list
shows no new source.  The BAT source is at low galactic latitude and has
high extinction.

We are currently in the portion of the orbits where the spacecraft does
not pass over the Malindi downlink station. Therefore, it will be ~6
hours before we have access to the full data set for the refined analyses.