GCN Circular 18507

P. Wiseman, J. Greiner (both MPE Garching) report on behalf of the GROND
team:

We observed the field of GRB 151027B (Swift trigger 661869; (Ukwatta et
al., GCN 18499
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 06:24 UT on 28/10/2015, 7 hrs 45 mins after the
GRB trigger, and are continuing. They were performed at an average seeing
of 0.9" and at an average airmass of 1.1.

We found a single point source within the enhanced 1.8" Swift-XRT error
circle reported by Evans et al. (GCN 18504
Not found
 
 
), common with that reported by
Malesani et al. (GCN 18501
Not found
 
 
) and Xu et al. (GCN 18505
Not found
 
 
).

Based on 6.6 min of total exposures in g'r'i'z'JHK at a mid time of
06:33:39 UT, we estimate preliminary magnitudes (all in AB system) of

g' > 22.4 mag,
r' = 20.8 +/- 0.08 mag,
i' = 20.1 +/- 0.07 mag,
z' = 20.1 +/- 0.07 mag,
J = 20.1 +/- 0.4 mag,
H = 19.1 +/- 0.3 mag, and
K > 18.3 mag.

A clear g'-band drop out in the SED leads to a photo-z of 4.3 + 0.3,-0.2,
consistent with the redshift derived from the X-shooter spectrum (Xu et
al., GCN 18505
Not found
 
 
). The source appears to be fading relative to the NOT
report (Malesani et al., GCN 18501
Not found
 
 
), and has continued to do so during our
observations.

Given magnitudes are calibrated against GROND zeropoints (g'r'i'z') as
well as 2MASS field stars (JHK) and are not corrected for the expected
Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.2
mag in the direction of the burst (Schlegel et al. 1998).