GCN Circular 38476

T. M. Parsotan (GSFC), A. D'Ai (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), R. Gupta (NASA GSFC), D. M. Palmer (LANL)
and A. Tohuvavohu (Caltech) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 05:35:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 241209A (trigger=1273071).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 155.376, +6.316 which is 
   RA(J2000) = 10h 21m 30s
   Dec(J2000) = +06d 18' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single short
peak structure with a duration of about 0.25 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 05:37:30.6 UT, 106.6 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 109 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 25% 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 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.024.

Although we do not yet have an XRT or UVOT counterpart, we believe this
is a true GRB.  It is high significance (>8 sigma) in the BAT image
and Fermi-GBM also detected the event.

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan AT nasa.gov).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)