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GCN Circular 23261

Subject
GRB 180924A: Swift detection of a burst
Date
2018-09-24T15:39:23Z (6 years ago)
From
Caryl Gronwall at PSU/Swift-UVOT <caryl@swift.psu.edu>
T. N. Ukwatta (LANL), A. P. Beardmore (U Leicester),
A. A. Breeveld (UCL-MSSL), P. A. Evans (U Leicester),
C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), M. H. Siegel (PSU)
and R. L. C. Starling (U Leicester) report on behalf of the Neil
Gehrels Swift Observatory Team:

At 15:22:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180924A (trigger=863186).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 49.161, -58.505 which is 
   RA(J2000) = 03h 16m 39s
   Dec(J2000) = -58d 30' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 130 sec.  The peak count rate
was ~1492 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 15:23:39.1 UT, 76.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
49.2002, -58.5321 which is equivalent to:
   RA(J2000)  = 03h 16m 48.05s
   Dec(J2000) = -58d 31' 55.6"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 122 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
http://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 2.03
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.35e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 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 19.6 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 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.03. 

This source lies within the current (Sector 2) field-of-view of TESS camera 3. 

Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta AT gmail.com). 
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.)
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