TITLE: GCN CIRCULAR NUMBER: 16191 SUBJECT: GRB 140428A: GROND observations DATE: 14/04/30 22:02:44 GMT FROM: Karla Varela at MPE F. Knust, K. Varela, J. Greiner (MPE Garching), and D. A. Kann (TLS Tautenburg) report on behalf of the GROND team: We observed the field of GRB 140428A (Swift trigger 597519; Kocevski et al., GCN #16177) 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 La Silla Observatory (Chile). Observations started at 00:43 UT on 2014-04-29, 2 hours after the GRB trigger. They were performed at an average seeing of 1.4" and at an average airmass of 1.9. We found a single point source within the 1.7" Swift-XRT error circle reported by Page et al. (GCN #16178) at RA (J2000.0) = 12h 57m 28.39s DEC (J2000.0) = + 28d 23' 06.4" with an uncertainty of 0.5" in each coordinate. This source is consistent with the one reported by Perley (GCN #16180). Based on the first 1133 s of total exposures in g'r'i'z' and 1920 s in JHK, we estimate preliminary magnitudes (all in AB system) of g' > 24.4 mag, r' = 23.6 +/- 0.2 mag, i' = 21.7 +/- 0.1 mag, z' = 21.5 +/- 0.1 mag, J > 19.7 mag, H > 20.6 mag, and K > 19.1mag. From our data, fading can not be established. We note that our r'-band magnitude is similar to that reported by Perley (GCN #16180) at a mid-time of 7.5 hrs after the GRB. The spectral energy distribution is best-fit by a straight power-law and a sharp cut-off below the i'-band which cannot be modelled with extinction alone. If this is the GRB afterglow, the cut-off corresponds to a photometric redshift of z = 4.8 +-0.3, consistent with the Keck spectroscopy as reported by Perley (GCN #16181). Given magnitudes are calibrated against SDSS (griz) as well as 2MASS (JHK) field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.01 mag in the direction of the burst (Schlegel et al. 1998).