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.