TITLE:   GCN CIRCULAR
NUMBER:  10399
SUBJECT: GRB 100205A: Keck limits on an underlying host galaxy
DATE:    10/02/10 13:06:10 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

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) 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).  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 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; Updike et al., GCN 10364; Cobb et al, GCN 10365)
* An apparently fading afterglow candidate detected only in H- and 
K-bands (Cucchiara et al., GCN 10374; Im et al., GCN 10398)
* Limited X-ray absorption: excess N_H(z=0) = 7(+6/-5) x 10^20 cm^-2 
(Starling and Racusin, GCN 10369), 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.