TITLE:   GCN CIRCULAR
NUMBER:  9720
SUBJECT: GRB 090715B: Late-time Lick 3m observations
DATE:    09/07/28 06:13:01 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, N. R. Butler, and S. B. Cenko (UC Berkeley) report:

We obtained additional, late-time imaging of the field of GRB 090715B 
(GCN 9668, Vetere et al.; GCN 9670, Smith et al.) with the Lick 3m 
telescope (+PFCam) in the R-band filter under dark skies, good seeing 
conditions and thin cloud cover.   Observations were conducted on the 
night of 2009-07-24 (UT) between 04:46 and 06:15 UT, and consisted of 
seven 300-second exposures followed by four 600-second exposures.

The afterglow is faintly detected in the combined image.  Photometry 
relative to the reference star of Malesani et al. (GCN 9671) gives a 
magnitude of R = 23.6 +/- 0.3 mag at this time (t = 8.35 days after the 
GRB).

Relative to the NOT observation at 2.06 days (Malesani et al., GCN 
9687), this corresponds to a decay index of alpha ~ 1.5, indicating that 
the unusually long period of slow fading has ended and the optical 
afterglow has resumed a "normal" power-law decay.