TITLE:   GCN CIRCULAR
NUMBER:  18317
SUBJECT: GRB 150915A: GROND detection of an optical counterpart candidate
DATE:    15/09/16 06:40:48 GMT
FROM:    Jochen Greiner at MPI  <jcg@mpe.mpg.de>

R. Yates, J. Bolmer and J. Greiner (all MPE Garching) report on behalf of
the GROND team:

We observed the field of GRB 150915A (D'Elia et al., GCN 18315) 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 23:28 UT on 2015-09-15, 2.2 hours after the GRB trigger.

Based on a 42 minutes exposure at a mid-time of 3.7 hours after the BAT 
trigger, at an average seeing of 1.6" and at an average airmass of 1.0, 
we detect an optical counterpart at the edge of the Swift-XRT error-circle at
  RA = 21 18 37.84
  Decl. = -34 54 48.3
(+/- 0.3")

at the following with magnitudes (AB system)

g' = 23.0 +/- 0.1 mag,
r' = 23.1 +/- 0.1 mag,
i' = 22.9 +/- 0.2 mag,
z' = 22.7 +/- 0.2 mag,
J  > 19.9 mag,
H  > 19.3 mag,
K  > 17.9 mag.

The given magnitudes are derived based on calibrating the images against
GROND zeropoints in the optical and 2MASS in the NIR, and are not corrected 
for the expected Galactic foreground extinction corresponding to a reddening 
of E_(B-V) = 0.10 mag in the direction of the burst (Schlegel et al. 1998).