TITLE:   GCN CIRCULAR
NUMBER:  9766
SUBJECT: Title: GRB 090809: Further NOT optical observations
DATE:    09/08/11 17:31:56 GMT
FROM:    Dong Xu at DARK,NBI  <dong@astro.ku.dk>

D. Xu, G. Leloudas, J. P. U. Fynbo, D. Malesani (DARK/NBI), P. Jakobsson 
(Univ. Iceland), and and G. Furesz (CfA/Harvard) report on behalf of a 
larger collaboration:

We continued to observe the field of GRB 090809 (Markwardt et al., GCN 
9754; Xu et al., GCN 9755; Landsman et al., GCN 9758) with the Nordic 
Optical Telescope equipped with ALFOSC.  We obtained 4x300 s i-band 
frames staring on August 10th,  22:25:35 UT, 28.9 hr after the burst.

The optical afterglow is still detected in the stacked frame. The 
magnitude is i=22.3+/-0.1 against several reference stars of the USNO B1 
catalog in the field. The improved magnitude in our previous observation 
is i=20.20+/-0.04, being consistent with the measurement in Wiersema et 
al. (GCN 9762).  Therefore, the optical afterglow is decaying with a 
power-law slope of \alpha=1.0, similar to the general decaying of the 
X-ray afterglow (Mao et al., GCN 9759).

We note that the rest-frame T90 (Barthelmy et al., GCN 9756) is less 
than 2 s for this burst at z=2.737 (Malesani et al., GCN9761). But the 
early energetic X-ray flares tend to make this burst more consistent 
with the class of the conventional long bursts.