[vsnet-grb-info 6891] GRB 081104: GROND Detection of the Optical/NIR Afterglow Candidate?

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8482
SUBJECT: GRB 081104: GROND Detection of the Optical/NIR Afterglow Candidate?
DATE:    08/11/05 11:23:12 GMT
FROM:    Jochen Greiner at MPI  <jcg at mpe.mpg.de>

C. Clemens, T. Kruehler, J. Greiner, R. Filgas, A. Yoldas (all MPE Garching),
A. Kupcu Yoldas (ESO) and G. Szokoly (Eoetvoes Univ., Budapest) 
report on behalf of the GROND team:
 
We observed the field of GRB 081104 (Swift trigger 333666; Parsons et al., GCN
#8473) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 
120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory
(Chile). 
  
Observations started at 07:43 UT on November 5th, 2008, 22.1 hr after the GRB
trigger. They were performed at an average seeing of 1.0" and at an average
airmass of 1.1.

We found a single point source at the edge of the astrometrically 
corrected 1.6'' Swift-XRT error circle reported by Beardmore et al. 
(GCN #8480) at
  
RA (J2000.0) = 06h 41m 57.13s  
DEC (J2000.0) = -54d 43' 09.9''
  
with an uncertainty of 0.5".
  
Based on the first 25 min of total exposures in g'r'i'z' and 20 min in
JHK, we estimate preliminary magnitudes (all in AB system) of
  
g' ~ 22.7 mag, 
r' ~ 22.6 mag, 
i' ~ 21.8 mag, 
z' ~ 21.7 mag, 
J ~ 21.2 mag,
H > 21.1 mag and
K > 20.0 mag

with errors of +/- 0.2. These magnitudes are calibrated against GROND
zeropoints as well as 2MASS field stars, and are not corrected for the
galactic foreground extinction of E(B-V)= 0.1
  
The extinction-corrected spectral energy distribution is only marginally
consistent with a power law, so at present (without proper calibration)
we cannot distinguish between a foreground star, an afterglow, or a host.
No statement about variability can be made at this point either. 

We note that a very faint object is also visible on the POSS-IIN plate at 
this location.