TITLE:   GCN CIRCULAR
NUMBER:  20822
SUBJECT: GRB 170306A: GROND observations
DATE:    17/03/06 15:17:58 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady@mpe.mpg.de>

Patricia Schady, Phil Wiseman (both MPE) and Jan Bolmer (ESO, Santiago) report:

We observed the field of GRB 170306A (Swift trigger 741220; Markwardt et al., GCN #20819) 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 07:20 UT on 2017-03-06, 14 minutes after the GRB trigger, and they continued for ~2 hours, until the start of astronomical twilight. They were performed at an average seeing of 1.3" and at an average airmass of 1.5.

Within the 3.7" XRT error circle (Markwardt et al., GCN #20819) we detect two uncatalogued sources in addition to the source reported by Guidorzi et al. (GCN #20820). These two newly identified sources have AB magnitudes r’~20.0 mag and r’~20.3 mag calibrated against GROND zeropoints and uncorrected for the significant Galactic foreground extinction along the line of sight. None of the sources within the XRT error circle show any significant variability during the 2 hours of GROND observations.

We acknowledge the excellent support provided by the Chilean observer at the telescope, and the support astronomer, Angela Hempel, in obtaining these data.