TITLE:   GCN CIRCULAR
NUMBER:  9001
SUBJECT: GRB 090313: Additional Gemini-S photometry
DATE:    09/03/15 20:00:06 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, S. B. Cenko, J. S. Bloom, W. Li, and B. E. Cobb (UC 
Berkeley) report:

We conducted additional imaging observations of the optical afterglow of 
  GRB 090313 (GCN 8979, GCN 8780) consisting of 2x90 seconds in g, r, i, 
and z filters on Gemini-S (GMOS) between 08:39 and 08:58 UT on 
2009-03-15 (47.7 hours after the burst).  At this time we measure an 
afterglow magnitude (calibrated using SDSS stars in the field) of r = 
21.77 +/- 0.05, confirming the steepening in the decay reported by de 
Ugarte Postigo et al. (GCN 8999).  The amount of fading relative to our 
first epoch (GCN 8997) is consistent across all four filters. We measure 
an average decay index (F ~ t^-alpha) of alpha=1.77 over this interval.