TITLE: GCN CIRCULAR NUMBER: 17729 SUBJECT: GRB 150423A: GROND observation DATE: 15/04/23 07:35:54 GMT FROM: Jochen Greiner at MPI K. Varela, F. Knust, J. Bodensteiner and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 150423A (Swift trigger 638808; Pagani et al., GCN #17728) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 06:32 UT on 2015-04-23, 4 min after the GRB trigger. They were performed at an average seeing of 0".8 and at an average airmass of 1.5. We detect a faint source within the 2.2" XRT error circle, at position RA (2000.0) = 14h 46m 18.9s Decl (2000.0) = +12d 17' 00" with an error of 0.5", about 1" away from the XRT centroid position. Based on 7.7 min of total exposure time in g'r'i'z' and 8.0 min in JHK, centered 24 min after the GRB, we estimate the following preliminary magnitudes (all in AB system): g' = 23.2 +/- 0.2 mag, r' = 23.1 +/- 0.2 mag, i' = 22.5 +/- 0.2 mag, z' = 22.0 +/- 0.2 mag, J = 19.3 +/- 0.4 mag, H > 18.4 mag, and K > 17.5 mag. No statement can be made about fading at this stage. Since there is a faint source also in the SDSS, this might be the host galaxy rather than the afterglow. Given magnitudes are calibrated against SDSS (g'r'i'z') as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.03 mag in the direction of the burst (Schlegel et al. 1998).