GCN Circular 10399

D. A. Perley, J. S. Bloom, S. B. Cenko, B. E. Cobb, A. N. Morgan, A. A. 
Miller, and C. R. Klein (UC Berkeley) report:

We conducted a deep optical imaging integration on the field of GRB 
100205A (Racusin et al., GCN 10361
Not found
 
 
) using LRIS on the Keck I 10m 
telescope starting at 09:48 UT on 2010 Feb 7.  We integrated for a total 
exposure time of 4000 seconds in g-band and 3770 seconds in R-band 
simultaneously under clear skies but variable and generally poor seeing 
conditions (1.5 arcsec average).  The mid-point of the integration was 
at 10:28 UT.

We detect no source at the position of the putative infrared afterglow 
(Tanvir et al., GCN 10366
Not found
 
 
).  Calibrating relative to USNOB1.0, we 
measure limiting magnitudes (3-sigma) of:

R > 26.7
g > 26.6

as measured at the afterglow location.  A faint source is marginally 
detected (<2 sigma) in R-band only at the northwestern edge of the XRT 
error circle (Evans et al., GCN 10367
Not found
 
 
), not consistent with the position 
of the infrared afterglow.

We also stacked the R-band and g-band images together to create a 
white-light composite image.  There is no significant detection in this 
image, with an approximate limiting magnitude of White > 27 mag.

All properties of this GRB are consistent with it being at high redshift:

* Lack of optical detection starting at early times (e.g. Malesani et 
al., GCN 10362
Not found
 
 
; Updike et al., GCN 10364
Not found
 
 
; Cobb et al, GCN 10365
Not found
 
 
)
* An apparently fading afterglow candidate detected only in H- and 
K-bands (Cucchiara et al., GCN 10374
Not found
 
 
; Im et al., GCN 10398
Not found
 
 
)
* Limited X-ray absorption: excess N_H(z=0) = 7(+6/-5) x 10^20 cm^-2 
(Starling and Racusin, GCN 10369
Not found
 
 
), generally suggestive of higher 
redshift (Grupe et al. 2007, AJ 133:2216G) and inconsistent with the 
alternative hypothesis of a large extinction column at low redshift for 
typical values of A_V/N_H.
* Lack of host galaxy to deep limits (Perley et al. 2009, AJ 138:1690)

As mentioned by Cucchiara et al., the implied redshift of this GRB if 
the red H-K color is due to Lyman-alpha absorption would be z ~ 11-13. 
However, significant dust absorption at intermediate redshift (e.g., 
A_V~3 mag at z~4) is still generally consistent with the available data, 
and unfortunately the X-ray afterglow is too faint to impose meaningful 
constraints on extinction in the K-band during the Gemini observation.

An image of the field is posted to:
http://lyra.berkeley.edu/~dperley/100205a/100205a_keck_gR.png

We encourage continued deep infrared follow-up of the field.