TITLE:   GCN CIRCULAR
NUMBER:  8257
SUBJECT: GRB 080916C: GROND Detection of the Optical Afterglow Candidate
DATE:    08/09/17 10:55:42 GMT
FROM:    Christian Clemens at MPE  <cclemens@mpe.mpg.de>

C. Clemens (MPE), A. Rossi (Tautenburg Obs.), J. Greiner and S. McBreen (both 
MPE) report on behalf of the GROND team:

GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI 
telescope at La Silla Observatory (Chile) observed the field of GRB 080916C, 
which triggered Fermi GBM (Goldstein et al., GCN #8245) and for which 
significant correlated emission was detected by the Fermi LAT (Tajima et al., 
GCN #8246). The field was followed up by Swift XRT (Kennea et al., GCN 
#8253).

Observations started at 07:57 UT on September 17th, 2008, 31.7 hr after the 
GBM trigger. They were performed at an average seeing of 1.5" and at an 
average airmass of 1.7 as well as under bright moon.

We found a faint source at the edge of the astrometrically corrected 2.1'' 
Swift-XRT error circle reported by Evans et al. (GCN #8255) at

RA (J2000.0) = 07h 59m 23.32s
DEC (J2000.0) = -56d 38' 18.0''

with an uncertainty of 0.5".

Based on the first 50 min of effective exposures, we estimate a preliminary 
magnitude of

z' ~ 21.74 mag +/- 0.17.

Given magnitude is calibrated against USNO-B1 field stars.

No statement about variability can be made at this point. Further observations 
are planned.

Please note, that no correction for the Galactic foreground reddening of 
E(B-V) = 0.32 mag (Schlegel et al., 1998) has been applied.