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GCN Circular 28165

Subject
GRB 200729A: Swift detection of a burst
Date
2020-07-29T20:10:34Z (4 years ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
P. A. Evans (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), M. H. Siegel (PSU) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 19:38:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200729A (trigger=984929).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 184.375, +45.582 which is 
   RA(J2000) = 12h 17m 30s
   Dec(J2000) = +45d 34' 56"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 40 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 19:40:25.5 UT, 140.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 184.37782, 45.59380 which
is equivalent to:
   RA(J2000)  = 12h 17m 30.68s
   Dec(J2000) = +45d 35' 37.7"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 43 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 (1.07 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.5
(+3.87/-3.22) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
303 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 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.01. 

The XRT position is within the extent of the nearby galaxy NGC 4242,
for which SIMBAD lists a redshift of 0.00176 (~7.4 Mpc). Follow up
observations are strongly encouraged. 

Burst Advocate for this burst is P. A. Evans (pae9 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/)
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