GCN Circular 17513

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

We observed the field of GRB 150301A (Swift trigger 632995; Lien et al.,
GCN #17510) 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 06:02 UT on 01-03-2015, 4.97 hrs after the GRB
trigger. They were performed at an average seeing of 1.4" and at an
average airmass of 1.5.

We found two point sources within the 5.0" Swift-XRT error circle reported
by Lien et al. (GCN #17510). The first source is at

RA (J2000.0) = 16 h 17 m 13.08 s
DEC (J2000.0) = -48d 42' 45.7"

The second source is at

RA (J2000.0) = 16 h 17 m 13.34 s
DEC (J2000.0) = -48d 42' 49.7"�

with an uncertainty of 0.3" in each coordinate for both sources.

Based on the first 50 min of total exposures in g'r'i'z' and 40 min in
JHK, we estimate the following preliminary magnitudes (all in AB system):

For the first source:

g' > 24.4 mag,
r' > 24.5 mag,
i' = 22.0 +/- 0.1 mag,
z' = 21.0 +/- 0.3 mag,
J = 18.8 +/- 0.2 mag,
H = 18.3 +/- 0.2 mag, and
K > 18.3 mag.

For the second source:

g' > 24.4 mag,
r' > 24.5 mag,
i' = 23.1 +/- 0.1 mag,
z' = 22.2 +/- 0.1 mag,
J = 19.6 +/- 0.3 mag,
H = 19.1 +/- 0.4 mag, and
K > 18.3 mag.

Both objects appear to be constant over the duration of our observation.
Therefore we suppose they are not related to the burst.

We note the field is crowded. We also notice the presence of a bright star
(~14 mag) west of the Swift-XRT error circle covering around 10% of the
encircled region.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=3.31 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).