TITLE:   GCN CIRCULAR
NUMBER:  7535
SUBJECT: GRB 080319B: Continued Gemini-South followup and possible host galaxy
DATE:    08/03/29 10:11:00 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, J. S. Bloom (UC Berkeley) and H.-W. Chen (U Chicago) report:

We have continued to monitor the optical afterglow of GRB 080319B from 
Gemini-South.  Additional images were taken on UT March 21, 25, and 28. 
  Despite the presence of the Moon and low elevation of the target, the 
afterglow remains detected in all filters.  We report on photometry, 
calibrated to SDSS DR6, as follows (including an improved analysis of 
images obtained on UT March 20, previously reported in GCN 7486).

t(hr)  filt mag   err
24.808  g   20.83 0.05
25.099  r   20.53 0.02
25.363  i   20.37 0.03
25.686  z   20.32 0.04

48.653  r   21.56 0.03

144.553 g   23.59 0.15
144.886 r   23.49 0.09
145.220 i   23.13 0.06
145.553 z   23.48 0.11

216.703 r   23.56 0.06
217.036 i   23.28 0.06

The object appears visibly extended at late times, with a possible 
projection extending to the south; this may be the host galaxy of this 
gamma-ray burst.  The imaging was acquired under variable seeing 
conditions and we caution that the presence of an underlying host may 
complicate the photometry.

While roughly consistent with simple power-law decay of alpha=1.2-1.3 
(e.g. GCN 7438, Li et al.), in detail the observations deviate slightly 
from a simple power-law, including a flattening in the last two sets of 
observations that may be due host galaxy light (and possibly a small 
amount of supernova contribution.)

In addition, we note the presence of a very red source at an offset of 
2.65" (slightly west of south) from the afterglow.  Preliminary 
photometry of this object gives the following:

g = 24.3  +/- 0.2  (marginal detection)
r = 23.76 +/- 0.06
i = 22.55 +/- 0.03
z = 21.71 +/- 0.03

This offset corresponds to 21 kpc at a redshift of 0.937 (Vreeswijk et 
al., GCN 7451).  It is dominated by a bright PSF-like center but also is 
visibly extended to the south.  It may be an intervening absorber.

Further follow-up is planned and encouraged.