GRB 090520

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.

A. Rossi (Tautenburg), T. Kruehler and J. Greiner (both MPE Garching) 
report on behalf of the GROND team:

GROND (Greiner et al. 2008, PASP 120, 405) at the 2.2m ESO/MPI telescope at La
 Silla Observatory (Chile) 
started follow-up observations of GRB 090520 (Cummings et al. 2009, GCN #9417
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 )
 on May 21, at 09:35 UTC 
(~32 h after the burst). In addition, a second epoch of imaging was obtained,
 which started on May 22, 09:37 UTC.

Image subtraction does not reveal any variable source between the two epochs. 
The first epoch thus yields the following upper limits:

g > 23.8
r > 24.0
i > 23.4
z > 23.2
J > 21.8
H > 21.4
K > 20.8

obtained using the GROND zeropoints and 2MASS field stars as reference.