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GCN Circular 22672

Subject
GRB 180425A: Swift detection of a burst
Date
2018-04-25T07:01:11Z (6 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
F. E. Marshall (NASA/GSFC), P. D'Avanzo (INAF-OAB),
P. A. Evans (U Leicester), H. A. Krimm (NSF/USRA),
A. Melandri (INAF-OAB), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:46:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180425A (trigger=828672).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 64.469, -32.956 which is 
   RA(J2000) = 04h 17m 53s
   Dec(J2000) = -32d 57' 20"
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 15 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 06:47:40.0 UT, 78.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 64.4517,
-32.9519 which is equivalent to:
   RA(J2000)  = 04h 17m 48.41s
   Dec(J2000) = -32d 57' 06.9"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 54 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 in excess of the Galactic value (2.90 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2
(+2.26/-2.02) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.37e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 87 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. 

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/too.html.)
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