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GCN Circular 9725

Subject
GRB 090728: Swift detection of a burst
Date
2009-07-28T15:00:04Z (15 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
P.A. Curran (MSSL-UCL), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC),
E. A. Hoversten (PSU), J. A. Kennea (PSU), N. P. M. Kuin (MSSL),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
P. Romano (INAF-IASFPA), T. Sakamoto (NASA/UMBC),
B. Sbarufatti (INAF-IASFPA), M. H. Siegel (PSU),
M. Stamatikos (NASA/ORAU) and M. A. Stark (PSU) report on behalf of
the Swift Team:

At 14:45:45 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090728 (trigger=358574).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 29.660, +41.647 which is 
   RA(J2000) = 01h 58m 38s
   Dec(J2000) = +41d 38' 49"
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 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 14:47:36.5 UT, 110.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 29.65234, 41.63270 which
is equivalent to:
   RA(J2000)  = 01h 58m 36.56s
   Dec(J2000) = +41d 37' 57.7"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 55 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
7.35e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 3.90e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
116 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 3-sigma upper limit is 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
0.07. The XRT position is near a bright (< 6.5 mag) star. 

Burst Advocate for this burst is V. Mangano (vanessa AT ifc.inaf.it). 
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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