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GCN Circular 9386

Subject
GRB 090518: Swift detection of a burst
Date
2009-05-18T02:09:07Z (15 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
P. A. Evans (U Leicester), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL),
A. Rowlinson (U Leicester), M. H. Siegel (PSU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 01:54:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090518 (trigger=352420).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 119.933, +0.768 which is 
   RA(J2000) = 07h 59m 44s
   Dec(J2000) = +00d 46' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 5 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 01:55:57.8 UT, 73.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 119.95519, 0.75892 which
is equivalent to:
   RA(J2000)  = 07h 59m 49.25s
   Dec(J2000) = +00d 45' 32.1"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 86 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 (4.65e+20
cm^-2, Kalberla et al. 2005), with an excess column of 4.9
(+3.56/-2.94) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 77 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 none of
the XRT error circle. 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 0.04. 

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/too.html.)
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