[vsnet-grb-info 7661] GRB 090423: GROND detection and preliminary photo-z

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9215
SUBJECT: GRB 090423: GROND detection and preliminary photo-z
DATE:    09/04/24 03:05:49 GMT
FROM:    Jochen Greiner at MPI  <jcg at mpe.mpg.de>

F. Olivares, T. Kruehler, J. Greiner, R. Filgas (all MPE Garching) report on 
behalf of the GROND team:

We observed the field of GRB 090423 (Swift trigger 350184, H. Krimm et 
al., GCN #9198) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m 
MPI/ESO telescope at La Silla Observatory (Chile). Observations started on 
23 Apr. 2009 at 23:08 UT, 15.2 h after the burst.

We detect the source within the XRT error circle (Evans et al. 2009, GCN #9205)
reported by Tanvir et al. (GCN #9202), Levan et al. (GCN #9206) and Cucchiara 
et al. (GCN# 9209) in the NIR channels.

Preliminary photometry yields the following magnitudes and upper limits in 
stacked images at a midtime of 16.7 h after the burst, obtained using 
SDSS/2MASS stars as reference:

Filter  Exp [s]    AB Mag   MagErr
----------------------------------
g'      16 x 369    > 25.0
r'      16 x 369    > 25.1
i'      16 x 369    > 24.2
z'      16 x 369    > 24.0
J       480 x 10    21.6     0.1
H       480 x 10    21.2     0.1
K       480 x 10    20.9     0.1

These magnitudes imply a significant fading to what was reported earlier 
(GCNs #9202, #9213) and confirm this source to be the afterglow of GRB 
090423. Furthermore, we confirm the red z-J color, and thus the probable 
high redshift nature of this event. After correcting for the foreground
galactic extinction of E(B-V)=0.029 (Schlegel et al. 1998) and assuming no
intrinisic extinction, a fit with Hyper-z (Bolzonella et al. 2000) results 
in a spectral index of 1.0 (+0.4,-0.5), comparable to that observed in 
X-rays (Stratta & Perri, GCN #9212), and a preliminary photometric redshift 
of z = 8.0+0.5-1.2.