GCN Circular 18252

P. Wiseman, C. Delvaux and J. Greiner (all MPE Garching) report on behalf
of the GROND team:

We observed the field of GRB 150907B (Swift trigger 654803; Roegiers et
al., GCN #18249) 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 02:03 UT on 08/09/2015, 2.63 hours after the GRB
trigger. They were performed at an average seeing of 2.5" and an average
airmass of 2.0.

We do not detect a source within the Swift-XRT error circle reported by
Osborne et al. (GCN #18251) down to limits of:

g' > 24.5 mag,

r' > 23.9 mag,

i' > 23.3 mag,

z' > 23.2 mag,

J > 21.1 mag,

H > 20.6 mag, and

K > 19.2 mag.

We note a source at the eastern edge of the Swift-XRT error circle, at a
position:
RA  =  255.3110
Dec = -63.7860,
or 17h 01m 14.64s -64d 47' 09.6".

This source does not seem to be fading, and with a non GRB-like SED, and
matches to the known source S7FP064671 from The Guide Star Catalogue,
Version 2.3.2 (Lasker B., Lattanzi M.G., McLean B.J., et al. Astron. J.
136, 735 (2008)).

The given limits are derived based on calibrating the images against GROND
zeropoints and 2MASS field stars and are not corrected for the Galactic
foreground extinction corresponding to a reddening of E_(B-V)= 0.1432 in
the direction of the burst (Schlegel et al. 1998).