TITLE:   GCN CIRCULAR
NUMBER:  37466
SUBJECT: GRB 240912A: Swift detection of a burst with an optical counterpart
DATE:    24/09/12 02:13:06 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

P. A. Evans (U Leicester), R. Gupta (NASA GSFC), J. A. Kennea (PSU),
K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 01:46:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240912A (trigger=1253910).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 124.633, +33.995 which is 
   RA(J2000) = 08h 18m 32s
   Dec(J2000) = +33d 59' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 150 sec, extending into
the observation times of the XRT and UVOT.  The peak count rate
was ~17000 counts/sec (15-350 keV), at ~69 sec after the trigger.

The XRT began observing the field at 01:48:25.2 UT, 106.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 124.63711,
33.99865 which is equivalent to:
   RA(J2000)  = 08h 18m 32.91s
   Dec(J2000) = +33d 59' 55.1"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 17 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (5.53 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.2
(+2.90/-2.49) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 8.64e-08 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 115 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =  08:18:32.40 = 124.63502
  DEC(J2000) = +33:59:56.3  =  33.99896
with a 90%-confidence error radius of about 0.75 arc sec. This position is 8.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.40 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.054.

Burst Advocate for this burst is P. A. Evans (pae9 AT leicester.ac.uk).
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/)