[vsnet-grb-info 12520] GRB 121024A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13886
SUBJECT: GRB 121024A: Swift detection of a burst with an optical counterpart
DATE:    12/10/24 03:12:57 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

C. Pagani (U Leicester), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), C. Gronwall (PSU), J. A. Kennea (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU) and
T. N. Ukwatta (MSU) report on behalf of the Swift Team:

At 02:56:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121024A (trigger=536580).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 70.467, -12.269 which is 
   RA(J2000)  =  04h 41m 52s
   Dec(J2000) = -12d 16' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a few overlapping peaks
with a duration of about 15 sec.  The peak count rate
was ~3500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 02:57:45.5 UT, 93.0 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 70.4715, -12.2904 which is equivalent to:
   RA(J2000)  = +04h 41m 53.16s
   Dec(J2000) = -12d 17' 25.4"
with an uncertainty of 5.5 arcseconds (radius, 90% containment). This
location is 78 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.10e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
158 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	04:41:53.28 =  70.47201
  DEC(J2000) = -12:17:26.8  = -12.29079
with a 90%-confidence error radius of about 0.78 arc sec. This position is 2.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.38 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.10. 

Burst Advocate for this burst is C. Pagani (cp232 AT star.le.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/too.html.)