[vsnet-grb-info 23861] GRB 191004B: GROND Detection of the Optical/NIR Afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25960
SUBJECT: GRB 191004B: GROND Detection of the Optical/NIR Afterglow
DATE:    19/10/06 10:24:52 GMT
FROM:    Patricia Schady at Swift  <p.schady at bath.ac.uk>

P. Schady (Univ. of Bath), J. Balmer (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.