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GCN Circular 28761

Subject
GRB 201024A: Swift detection of a burst with an optical afterglow
Date
2020-10-24T03:10:12Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
F. E. Marshall (NASA/GSFC), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), N. J. Klingler (PSU),
N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB)
and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 02:48:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 201024A (trigger=1001514).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 125.948, +3.377 which is 
   RA(J2000) = 08h 23m 47s
   Dec(J2000) = +03d 22' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 5 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 02:50:14.0 UT, 74.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 125.9521,
3.3537 which is equivalent to:
   RA(J2000)  = 08h 23m 48.50s
   Dec(J2000) = +03d 21' 13.4"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 85 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 3.64
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 6.41e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 495 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	08:23:48.55 = 125.95229
  DEC(J2000) = +03:21:14.5  =	3.35403
with a 90%-confidence error radius of about 0.62 arc sec. This position is 0.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.85 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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/)
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