GCN Circular 23326

P. Schady (Uni. of Bath, UK) and J. Bolmer (MPE, Garching) report:


We observed the field of GRB 181010A (Swift trigger 866434; Melandri et al., GCN #23309) 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 ESO La Silla Observatory (Chile).


Observations were performed in rapid response mode, and started at 05:59 UT, 3 mins after the GRB trigger. They were performed at an average seeing of 1.4" and at an average airmass of 1.0.


We detect a new source at a position consistent with the X-ray and optical afterglow (Gorbovskoy et al., GCN #23310; Evans et al., GCN #23313; Emery, GCN #23317). Based on the first 3.3 min of total exposures in g'r'i'z' and 3 min in JHK, we measure the following preliminary AB magnitudes:


g' = 20.02 +/- 0.02 mag
r' = 19.33 +/- 0.01 mag
i' = 19.13 +/- 0.02 mag
z' = 18.56 +/- 0.01 mag
J = 17.78 +/- 0.03 mag
H = 17.02 +/- 0.03 mag


Given magnitudes are calibrated against PanSTARRS and 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.03 in the direction of the burst (Schlafly & Finkbeiner 2011).


We thank Markus Rabus for the excellend support from La Silla.