TITLE: GCN CIRCULAR NUMBER: 6774 SUBJECT: GRB 070809: Confirmation of optical transient DATE: 07/09/08 22:39:45 GMT FROM: Daniel Perley at U.C. Berkeley D. A. Perley (UC Berkeley), C. C. Thoene (DARK, UCB), and J. S. Bloom (UCB) report: After our initial observations (GCN 6739) of the possibly short GRB 070809 (Palmer et al., GCN 6728), we obtained a second series of imaging the following night using the Keck I telescope (+LRIS). We imaged the field for a total of 640s (R) and 880s (g) simultaneously in a series of images starting at 06:20:17 UT (2007-08-11) at very high airmass. The point-like source reported in GCN 6739 visible the first night has significantly faded, and is detectable only at the 3 sigma level in R-band and is undetected in g-band. Using an absolute calibration relative to Landolt standard stars, we calculate magnitudes of this source of: UT (mid-time) t_burst (hr) magnitude 2007-08-10 06:35 11.21 R = 24.1 +/- 0.2 g = 25.7 +/- 0.2 2007-08-11 06:31 35.14 R = 25.0 +/- 0.3 g > 26.0 (3-sigma) We caution that the source is very close to a bright (R~13) star and slightly blended with a diffraction spike in our imaging. This and the very high airmass of the observations may create additional uncertainties in the photometry. In addition, we acquired a 2x600s spectrum of the nearby edge-on galaxy noted in GCN 6739 during twilight starting at 05:46 UT (2007-08-11), covering the wavelength range from about 3800 to 9200 AA. No obvious emission lines are detected.