GCN Circular 16977

J. F. Graham (MPE Garching), D. A. Kann (TLS Tautenburg), P. Schady, and
J. Greiner (both MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 141028A (Fermi trigger 436186489; LAT
detection: Bissaldi et al., GCN #16969
Not found
 
 
; GBM Detection: Roberts; GCN
#16971
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 23:57 UT on October 28th, approximately 13 hours
after the GRB trigger. They were performed at an airmass of 1.2 and seeing
of 1".4.

At the position of the MASTER candidate afterglow (Gorbovskoy, GCN #16972
Not found
 
 
)
we detect a bright source at

RA (J2000.0) = 322.60226 = 21:30:24.54
DEC (J2000.0) = -0.231314 = -00:13:52.7

with an error of 0".5.

Based on images with an exposure time of 1500 s in the optical and 1200 s
in the NIR, centered at 0.55305 days after the trigger, we estimate
preliminary magnitudes and limits (all in AB system) for this source of:

g' = 20.76 +/- 0.02 mag
r' = 19.93 +/- 0.01 mag
i' = 19.70 +/- 0.02 mag
z' = 19.49 +/- 0.02 mag
J = 19.24 +/- 0.11 mag
H = 18.94 +/- 0.08 mag
K > 18.3 mag

Compared to the MASTER detection, the source has faded significantly. We
also detect a fading by 0.1 +/- 0.03 mag over the time span of an hour.
Furthermore, the SED is adequately described by a power-law of slope beta
= 1.1, with a possible g'-band dropout, which may point to a redshift of
around 3 (which would make this a luminous afterglow).

At the position of the XRT source reported in the GRB_COUNTERPART POSITION
NOTICE (Source 3 at http://www.swift.ac.uk/xrt_products/TILED_GRB00030/),
we detect no source down to limiting magnitudes of:

g' > 24.5 mag
r' > 24.4 mag
i' > 23.7 mag
z' > 23.4 mag
J > 20.9 mag
H > 20.6 mag
K > 18.3 mag

We therefore deem it likely the MASTER source represents the afterglow of
GRB 141028A.

Given magnitudes are calibrated against SDSS field stars (g'r'i'z')
and 2MASS field stars (JHK) and are not corrected for the expected
Galactic foreground extinction corresponding to a reddening of E_(B-
V) = 0.05 mag in the direction of the burst (Schlegel et al. 1998).