TITLE:   GCN CIRCULAR
NUMBER:  10374
SUBJECT: GRB 100205A: Gemini-N + NIRI photometry
DATE:    10/02/05 21:18:42 GMT
FROM:    Antonino Cucchiara at PSU  <cucchiara@astro.psu.edu>

A. Cucchiara (PSU), D. B. Fox (PSU), N. R. Tanvir (U. Leicester),
A. J. Levan (U. Warwick), B. E. Cobb (UC Berkeley), E. Berger 
(Harvard/CfA),
and R. Chornock (Harvard/CfA) report on behalf of a larger collaboration:

We have calibrated the JHK photometry for the candidate Gemini-N + NIRI
afterglow (Tanvir et al., GCN 10366) of GRB 100205A (Racusin et al.,
GCN 10361) against two 2MASS stars in the field.  Converted to AB 
magnitudes we find:

J_AB > 23.37 mag (1-sigma UL)
H_AB     = 23:54 +- 0.22 mag
K_AB     = 21.91 +- 0.20 mag

Thus, based on our photometry we measure a red K-H color, K_AB - H_AB = 
1.6 +- 0.5 mag.
If this red color is due to Lyman-alpha absorption within the H filter 
bandpass, then
this would imply a redshift of 11 <~ z <~ 13.5.   In this connection, we 
note that we are
retracting our earlier claim of a J-band detection (Tanvir et al., GCN 
10366) at the
candidate position.

Lower-redshift solutions with significant local (host) extinction, 
unusual afterglow
colors, or substantial contributions from an underlying host galaxy 
cannot be
excluded at this time.