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GCN Circular 9198

Subject
GRB 090423: Swift detection of a burst
Date
2009-04-23T08:14:38Z (15 years ago)
From
Hans Krimm at NASA-GSFC <krimm@milkyway.gsfc.nasa.gov>
H. A. Krimm (CRESST/GSFC/USRA), A. P. Beardmore (U Leicester),
P. A. Evans (U Leicester), O. Godet (U Leicester), C. Gronwall (PSU),
C. Guidorzi (U Ferrara), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. Perri (ASDC),
B. Sbarufatti (INAF-IASFPA), P. Schady (MSSL-UCL), G. Stratta (ASDC),
G. Tagliaferri (INAF-OAB) and T. N. Ukwatta (GSFC/GWU) report on
behalf of the Swift Team:

At 07:55:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090423 (trigger=350184).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 148.895, +18.160 which is 
   RA(J2000) = 09h 55m 35s
   Dec(J2000) = +18d 09' 37"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 07:56:31.8 UT, 72.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 148.8879,
18.1497 which is equivalent to:
   RA(J2000)  = 09h 55m 33.08s
   Dec(J2000) = +18d 08' 58.9"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 44 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
2.89e+20 cm^-2 (Kalberla et al. 2005). 


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 25% of
the BAT 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
BAT 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 H. A. Krimm (krimm 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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