GCN Circular 20843
Patricia Schady (MPE Garching) reports: Following on from the GROND (Greiner et al. 2008, PASP 120, 405) observations of GRB 170306A on the 2017-03-06 (Schady et al., GCN #20819), I re-observed the field simultaneously in g'r'i'z'JHK with GROND at on 2017-03-10, 4 days after the GRB trigger. Observations were performed at an average seeing of 1.4" and at an average airmass of 1.7. One of the three sources detected with GROND (Schady et al., GCN #20819) within the refined XRT error circle (D�Avanzo et al., GCN #20828) is no longer seen in the latest GROND observations down to the following 3 sigma limiting magnitudes (all in AB system): g� > 22.6 mag r� > 23.5 mag i� > 22.3 mag z� > 22.4 mag J > 20.4 mag H > 19.9 mag K > 18.9 mag This source is therefore the likely optical/NIR afterglow of GRB 170306A. The position of the afterglow candidate is RA (J2000) = 17:32:16.69 Dec (J2000) = -44:44:55.5 with a positional uncertainty of 0.3" in both coordinates. Based on combined images with 36 min of total integration time in g'r'i'z' and 30 min in JHK at a mid-time of 09:33 UT on 2017-03-06, the afterglow candidate had the following preliminary magnitudes (all in AB system): g' = 21.6 +/- 0.1 mag r� = 20.4 +/- 0.1 mag i� = 20.0 +/- 0.1 mag z' = 19.9 +/- 0.1 mag J = 19.1 +/- 0.1 mag H = 18.4 +/- 0.1 mag K = 18.5 +/- 0.2 mag The given magnitudes are derived based on images calibrated against GROND zero points in the optical and 2MASS field stars in the NIR bands. They are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.36 in the direction of the burst (Schlafly & Finkbeiner 2011). I acknowledge the excellent support provided by the Chilean observer at the telescope, and the support astronomer, Sam Kim, in obtaining these data. I also thank David Buckley for some useful email exchanges that in part led to the second epoch of GROND observations.