GCN Circular 25960

P. Schady (Univ. of Bath), J. Bolmer (MPE Garching), and A. Nicuesa Guelbenzu (TLS Tautenburg) report:


We observed the field of GRB 191004B (Swift trigger 927839; Cenko et al., GCN #25947) 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 started at 02:25 UT on 5th October, 4.9 hours after the GRB trigger. They were performed at an average seeing of 1.5" and at an average airmass of 1.8.


We detect a single point source consistent with the position of the Swift-XRT and MASTER afterglow (Beardmore et al., GCN # 25951; Lipunov et al., GCN # 25948) in all optical and NIR bands apart from K. In an 8 min exposure, we measure an AB magnitude in the r� band filter of r' = 20.86 +/- 0.04 mag.


The optical and NIR afterglow is still clearly detected in a second epoch of observations taken on the night of 6th October, with an AB magnitude r' = 22.10 +/- 0.03 mag.

The above magnitudes are calibrated again sky-mapper, and not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.01 in the direction of the burst (Schlafly & Finkbeiner 2011).

We acknowledge the excellent support from Regis Lachaume in La Silla in acquiring these observations.

[GCN OPS NOTE(29apr20): Per D.A.Kann, the spelling of the "Balmer" author was changed to "Bolmer".]