GCN Circular 5581

H. Krimm (GSFC/USRA), S. Barthelmy (GSFC), S. Campana (INAF-OAB),
J. Cummings (GSFC/UMBC), G. Israel (INAF-OAR), D. Palmer (LANL),
A. Parsons (GSFC) on behalf of the Swift-BAT team:

The Burst Alert Telescope (BAT) on Swift has detected a faint outburst from
the anomalous X-ray pulsar (magnetar) CXOU J164710.2-455216 in the cluster
Westerlund 1.  This pulse was 20 msec long starting at 01:34:52 UTC
Sep. 21, 2006 and shows a FRED profile.    The best fit postion is
RA, Dec 251.749, -45.918 (J2000), +/- 3 arcmin  (radius, sys+stat,
90% containment).  The Chandra position of the AXP is just outside the
error circle, at 3.3 arc minutes from the best position.

The transient event is detectable in the 12-60 keV band at 0.084 cts/cm2.
Fitting to a simple power law gives a fluence in the 15-150 keV band of
1.1 +/- 0.6 X 10^-7 erg/cm2 and a power law index 3.5 +/- 0.5.  All the
quoted errors are at the 90% confidence level.

This location was observed with the XRT as a target of opportunity starting
at approximately 14:29 UTC Sep. 21, 2006.  The spacecraft is currently in
the data gap so the XRT data will not be avaiable for at least another
two hours.

Note that this trigger generated a GCN SWIFT_BAT_POSITION and a
SWIFT_BAT_TRANSIENT_POSITION both of which were retracted based on
examination of the TDRSS light curve.  It was identified in the flight
code as coincident with GX 340+01, which is 22 arc minutes away.
More careful analysis of the ground data and a close positional coincidence
with CXOU J164710.2-455216 indicates that BAT detected a faint but real
outburst, and it was probably from this transient source.

We encouraged follow-up observations of the source.