TITLE: GCN CIRCULAR NUMBER: 19975 SUBJECT: GRB 161001A: GROND afterglow candidate DATE: 16/10/01 17:51:22 GMT FROM: Ting-Wan Chen at MPE T.-W. Chen (MPE Garching), S. Klose, A. Nicuesa Guelbenzu (TLS), J. Bolmer (MPE Garching and ESO Vitacura), T. Kruehler and J. Greiner (both MPE Garching) report on behalf of the GROND team: We observed the field of GRB 161001A (Swift trigger 714404; Page et al., GCN #19967) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla Observatory (Chile). Observations started at 02:48 UT on 2016-10-01, 1.71 hours after the GRB trigger. They were performed at an average seeing of 2" and at an average airmass of 2.3. We found a faint singular point source which is south-west 2.2 arcsec away from the photometric center of nearby bright DSS2 object for which Kruehler et al. (GCN #19971) reported X-Shooter spectroscopy. The position is within the 2.3" Swift-XRT error circle reported by Evans et al. (GCN #19968) and also within the 1.9" enhanced XRT position (Osborne et al., GCN 19969), at RA (J2000.0) = 04 h 47 m 40.57 s DEC (J2000.0) = -57d 15' 39.2 with an uncertainty of 0.5" in each coordinate. Based on images with 36 min of total exposure in g'r'i'z' and 30 min in JHK at a mid-time of 03:58 UT on 2016-10-01, we derive the following preliminary magnitudes and upper limits (all in the AB system): g' = 23.1 +- 0.2 mag, r' = 22.5 +- 0.1 mag, i' = 21.6 +- 0.1 mag, z' = 21.5 +- 0.1 mag, J > 21.3 mag, H > 20.3 mag, and K > 18.7 mag. Second-epoch observations were performed at a mean time of 09:19 UT at an average seeing 1.1" and at an average airmass of 1.1. At that time the source seems to have faded to: r' = 22.9 +- 0.1 mag Given magnitudes are calibrated against USNO as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.01 mag in the direction of the burst (Schlafly & Finkbeiner 2011).