GCN Circular 17522

D. A. Kann (TLS Tautenburg), C. Delvaux and J. Greiner (both MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 150301B (Swift trigger 633180; Lien et al.,
GCN #17515) 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 00:09 UT on 2015-03-02, 4.5 hrs after the GRB
trigger. They were performed at an average seeing of 1".0 and at an
average airmass of 1.1.

We detect the optical afterglow candidate reported by Lien et al. (GCN
#17515) and Buckley et al. (GCN #17518).

Based on 7.7 min of total exposure time in g'r'i'z' and 8.0 min in JHK, 
centered 5 hrs after the GRB, we estimate the following preliminary
magnitudes (all in AB system):

g' = 21.7 +/- 0.2 mag,
r' = 21.4 +/- 0.1 mag,
i' = 21.2 +/- 0.1 mag,
z' = 21.0 +/- 0.1 mag,
J = 20.5 +/- 0.3 mag,
H = 20.6 +/- 0.4 mag, and
K > 17.9 mag.

Given magnitudes are calibrated against GROND zeropoints (g'r'i'z') as
well as 2MASS field stars and are not corrected for the expected Galactic 
foreground extinction corresponding to a reddening of E_(B-V)=0.08 mag in
the direction of the burst (Schlegel et al. 1998).