GCN Circular 9215
Subject
GRB 090423: GROND detection and preliminary photo-z
Event
Date
2009-04-24T03:05:49Z (17 years ago)
From
Jochen Greiner at MPI <jcg@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.