[AAVSO-HEN] GRB 080913: GROND observation of a high-z optical/NIR afterglow candidate

[grb at aavso.org]

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TITLE:   GCN CIRCULAR
NUMBER:  8218
SUBJECT: GRB 080913: GROND observation of a high-z optical/NIR afterglow candidate
DATE:    08/09/13 09:07:05 GMT
FROM:    Jochen Greiner at MPI  <jcg at mpe.mpg.de>

A. Rossi (Tautenburg Obs.), J. Greiner, T. Kruehler, A. Yoldas (all MPE), 
S. Klose (Tautenburg Obs.), A. Kuepcue Yoldas (ESO)
report on behalf of the GROND team: 

We observed the field of GRB 080913 (Swift trigger 324561; Schady et
al., GCN #8217), simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2m ESO/MPI telescope at La Silla 
Observatory (Chile). Observations started at 06:50 UT on September 13, 2008, 
3 min after the GRB trigger. We detect a faint optical object inside
the 2.4 arcsec Swift/XRT error box (Schady et al., GCN #8217).

We detect the object only in the z'-band and NIR, but not in i'-band
or bluer. Preliminary photometry yields the following magnitudes calibrated 
against USNO-B1 and 2MASS field stars in stacked images: 
r' > 23.2 mag,
i' > 23.0 mag,
z' = 22.4 mag,
not corrected for the expected Galactic foreground reddening of E(B-V) =
0.05 mag (Schlegel et al., 1998). 

The non-detection in i' and bluer may indicate a redshift of about 6.
We caution however, that the drop-out is only 0.6 mag in depth between
our i' and z' bands, and our lack of better sensitivity could make this
a wrong interpretation. We encourage deeper observations.