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GCN Circular 36556

Subject
GRB 240529A: Swift detection of a burst with an optical counterpart
Date
2024-05-29T03:38:03Z (2 months ago)
From
David Palmer at LANL <palmer@lanl.gov>
Via
email

R. A. J. Eyles-Ferris (U Leicester), R. Gupta (NASA/GSFC),
A. Y. Lien (U Tampa), K. L. Page (U Leicester), D. M. Palmer (LANL)
and T. M. Parsotan (GSFC) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 02:58:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240529A (trigger=1231488).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 335.322, +51.555 which is 
   RA(J2000) = 22h 21m 17s
   Dec(J2000) = +51d 33' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of at least 20 sec since we are missing data 
after ~8 sec.  The peak count rate was ~4000 counts/sec (15-350 keV), 
at ~0 sec after the trigger. 

The XRT began observing the field at 03:06:22.0 UT, 471.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 335.35696, 51.56207 which
is equivalent to:
   RA(J2000)  = 22h 21m 25.67s
   Dec(J2000) = +51d 33' 43.5"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 82 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 consistent with the Galactic value of 4.06
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 128 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	22:21:26.04 = 335.35851
  DEC(J2000) = +51:33:43.4  =  51.56205
with a 90%-confidence error radius of about 1.10 arc sec. This position is 2.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.84. No correction has been made for the large, but uncertain, extinction
expected. 

Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 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/)


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