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GCN Circular 9254

Subject
GRB 090426: Swift detection of a burst with an optical counterpart
Date
2009-04-26T13:02:36Z (15 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
J. R. Cummings (NASA/UMBC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), H. A. Krimm (CRESST/GSFC/USRA),
J. Mao (INAF-OAB), R. Margutti (Univ Bicocca&OAB),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), B. A. Rowlinson (U Leicester),
M. H. Siegel (PSU) and T. N. Ukwatta (GSFC/GWU) report on behalf of
the Swift Team:

At 12:48:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090426 (trigger=350479).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 189.091, +32.966 which is 
   RA(J2000) = 12h 36m 22s
   Dec(J2000) = +32d 57' 56"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple narrow
peaks extending over 25 seconds.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:50:11.8 UT, 84.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 189.07405, 32.98476 which is equivalent
to:
   RA(J2000)  = 12h 36m 17.77s
   Dec(J2000) = +32d 59' 05.1"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 84 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
1.51e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 89 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	12:36:18.07 = 189.07531
  DEC(J2000) = +32:59:09.6  = 32.98600
with a 90%-confidence error radius of about 0.75 arc sec. This position is 86.2
arc sec. from the center of the BAT error circle. The estimated magnitude is
17.50 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is J. R. Cummings (jayc 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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