TITLE: GCN CIRCULAR NUMBER: 15080 SUBJECT: GRB 130623A: GROND detection of the optical/NIR afterglow DATE: 13/08/07 08:35:34 GMT FROM: Karla Varela at MPE K. Varela (MPE Garching), A.C. Updike (Roger Williams University), J. Elliott (MPE Garching), J. Greiner (MPE Garching) and D.A. Kann (TLS Tautenburg) report on behalf of the GROND team: We observed the field of GRB 130623A (Cummings et al., GCN #14921) simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile). The first epoch observation started at 09:15 UT on 25 June 2013, 33.7 hrs after the GRB trigger. They were performed at an average seeing of 1."5 and at an average airmass of 1.6. We find a single point source at the edge of the 4."5 Swift-XRT error circle reported by Grupe et al. (GCN #14926) at RA (J2000.0) = 01:23:26.17 = 20.859063 DEC (J2000.0) = -77:46:24.0 = -77.773361 with an uncertainty of 0.3" in each coordinate. Based on a total exposure of 50 minutes in g'r'i'z' and 40 minutes in JHK, we estimate preliminary magnitudes (all in AB) of g' = 22.5 +/- 0.1 mag, r' = 22.4 +/- 0.1 mag, i' = 22.2 +/- 0.1 mag, z' = 21.9 +/- 0.1 mag, J = 21.3 +/- 0.3 mag, H = 20.5 +/- 0.3 mag, and K > 20.2 mag. The second epoch observations started at 08:29 UT on 05 August 2013, 42 days after the GRB trigger. They were performed at an average seeing of 1."3 and at an average airmass of 1.5. The source found in the first epoch is not detected anymore: Based on a total exposure of 75 minutes in g'r'i'z' and 60 minutes in JHK, we estimate preliminary upper limits (all in AB) of g' > 25.7 r' > 25.2 i' > 24.3 z' > 24.0 J > 21.8 H > 20.9 Ks > 20.4 The fading by nearly 3 mag in the r'-band proves it to have been the afterglow of GRB 130623A. The clear detection down to the g'-band implies a redshift smaller than 3. Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.05 mag in the direction of the burst (Schlegel et al. 1998).