GCN Circular 17336

P Wiseman, J Graham, P Schady and J Greiner (All MPE Garching) report on
behalf of
the GROND team:

We observed the field of GRB 150120B (Swift trigger 627170; D'Elia et al.,
GCN 17314
Not found
 
 
) 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 01:25 UT on 21.01.2015, ~18hrs after the GRB
trigger. They were performed at an average seeing of 1.1" and at an
average airmass of 1.7.

We find a single point source within the 1.4" Swift-XRT error circle (Goad
et al., GCN 17320
Not found
 
 
), consistent with that
reported by Perley et al. (GCN 17315
Not found
 
 
), Guidorzi et al. (GCN 17316
Not found
 
 
), at

RA (J2000.0) = 02 h 37 m 09.77 s

DEC (J2000.0) = +08d 04' 40.49"

with an uncertainty of 0.3" in each coordinate.

Based on 25 min of total exposures in g'r'i'z'JHK, we estimate preliminary
magnitudes and 3 sigma upper limits (all in AB system) of

g' > 24.2
r' = 23.6 +/- 0.1 mag,
i' = 22.7 +/- 0.2 mag,
z' = 22.4 +/- 0.40 mag,
J = 20.2 +/- 0.3 mag,
H = 19.9 +/- 0.5 mag, and
K = 18.7 +/- 0.4 mag.

Based on these magnitudes, the best-fit SED has a break in the g' band
suggesting a photometric redshift of 3.5 +/-0.5.

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)=0.17 mag in the
direction of the burst (Schlegel et al. 1998).