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 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.