[vsnet-grb-info 7873] GRB 090520 Swift-BAT detection of a burst in ground analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9417
SUBJECT: GRB 090520 Swift-BAT detection of a burst in ground analysis
DATE:    09/05/21 21:04:47 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <jayc at milkyway.gsfc.nasa.gov>

J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), E. E. Fenimore (LANL),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):

On May 20, 2009 at 01:37:16 UT, BAT triggered on GRB 090520
(trigger #352671).  The burst was weak, and the instrument 
failed to find a source in onboard analysis.  A significant 
source was found in ground analysis.  The source is located at 
RA, Dec 11.613, -8.000 deg, which is

   RA(J2000)  =  00h 46m 27.1s
   Dec(J2000) = -08d 00' 00"
with an uncertainty of 3 arcmin (radius, sys+stat, 90% 
containment). The partial coding was 48%.

The burst consisted of a single FRED pulse, starting about
T-2 seconds, with a T90 of 20 ± 5 seconds (estimated error 
including systematics).

The BAT event data for failed triggers is limited to 10 seconds.  
Using 10 seconds of event data from T-2 to T+8 seconds, the
best fit to the spectrum is a simple powerlaw with a photon
index of 0.8 +- 0.2.  The fluence for this interval in the
15-150 keV band was 3.4 ± 0.5 x 10-7 ergs/cm2 
(68% confidence).

The BAT source position was about 3 arcmin from a low-redshift
galaxy, 6dFGS gJ004613.4-080000.  This is an emission-line 
galaxy at z = 0.0850.

A Swift TOO was requested, but initially the source was in the
Swift Moon constraint.  In any case was 52 degrees (~2.5 hours 
in RA) from the Sun, not in constraint, but still an unfavorable 
angle, so no Swift followup observation is planned.