[vsnet-grb-info 7714] GRB 090426: GROND Observations of the Optical/NIR Afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9268
SUBJECT: GRB 090426: GROND Observations of the Optical/NIR Afterglow
DATE:    09/04/27 03:28:00 GMT
FROM:    Aybuke Kupcu Yoldas at ESO  <ayoldas at eso.org>

F. Olivares (MPE Garching), A. Kupcu Yoldas (ESO), J. Greiner, A. Yoldas 
(both MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 090424 (Swift trigger 350479; Cummings et 
al., GCN #9254) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 
2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla 
Observatory (Chile).

Observations started at 01:08 UT on 27 April, ~12.5 hours after the GRB 
trigger, and are continuing. They are performed at an average seeing of 
1.2 and at an average airmass of 2.5.

We clearly detect the optical/near-infrared afterglow (Cummings et al. 
GCN #9254; Xin et al. #GCN 9255; Oates et al. GCN #9265; Yoshida et al. 
GCN #9266, #9267) in all GROND bands.

Preliminary photometry yields the following AB magnitudes in stacked 
images, obtained using SDSS/2MASS stars as reference:

Filter  Exp[s]  AB Mag  MagErr
----------------------------------------
g'      4x115	22.04	0.03
r'      4x115	21.24	0.24
i'      4x115	21.77	0.11
z'      4x115	21.43   0.09
J 	48x10	20.40	0.10	
H 	48x10	20.09	0.12
K 	48x10	19.83	0.17

Given magnitudes are not corrected for the expected Galactic foreground 
extinction corresponding of E_(B-V)=0.02 mag (Schlegel et al. 1998).

Using these magnitudes combined with the results of previous 
observations (Yoshida et al. GCN #9266) we estimate a simple power law 
decay with an index of ~0.86.