TITLE: GCN CIRCULAR NUMBER: 10652 SUBJECT: GRB 100423A: GROND discovery of afterglow DATE: 10/04/23 01:42:03 GMT FROM: Jochen Greiner at MPI Robert Filgas, Jochen Greiner (both MPE) and Adria Updike (Clemson Univ.) report on behalf of the GROND team: We observed the field of GRB 100423A (Swift trigger 420247; Stroh et al., GCN 10651) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 00:37 UT on April 23, 2010, 150 sec after the Swift trigger. They were performed at an average seeing of 1" and at airmass of 1.8, respectively. We found a single variable point source within the 3' error circle, not visible in either 2MASS or DSS2 at RA (J2000.0) = 09h 05m 50.22s Dec (J2000.0) = +21d 29' 54.3" with an uncertainty of 0.3" in each coordinate. From the first 4 min observation we estimate the following preliminary magnitudes g = 17.7+-0.1 r = 17.6+-0.1 i = 17.5+-0.1 z = 17.4+-0.1 J = 17.3+-0.1 H = 16.9+-0.1 K = 16.5+-0.1 calibrated against SDSS and 2MASS, and not corrected for the expected foreground extinction of E(B-V)=0.04 (Schlegel et al. 1998). The object faded by 0.3 mag between the first and second 4 min observation, and thus is likely the GRB afterglow. Since the object is seen in all GROND bands, the redshift is smaller than about 3.5. We encourage spectroscopic observations.