D. Xu, G. Leloudas, D. Malesani, J. P. U. Fynbo, J. Maund (DARK/NBI), P.
Jakobsson (Univ. Iceland), and T. Liimets (NOT & Tartu Obs.) report on
behalf of a larger collaboration:
We continued to observe the field of GRB 090529 (Sakamoto et al., GCN
9430) with the Nordic Optical Telescope equipped with ALFOSC. We
obtained 3x600 s R-band frames staring on June 01, 21:26:31 UT and
ending on June 01, 22:00:52 UT. The mid-exposure time is 3.31 days after
The optical afterglow (Xin et al., GCN 9431; Schady et al., GCN 9435) is
still detected in the stacked frame. Using the same reference star as in
Kann et al. (GCN 9436) and in Leloudas et al. (GCN 9453), we obtain
R=22.8+/-0.1 for the afterglow.
Measurements by Kann et al. (0.3 day after the burst) and Leloudas et
al. (1.29 days after the burst) indicate an optical power-law decay
index of alpha1=0.72. Our new observation indicates that the optical
decay became even slower, with an index of alpha2=0.3 between our two
obs epochs. In contrast, during the whole period of 0.3 - 3.31 days the
X-ray afterglow decays with the index of ~1.0 (ref, Evans et al. 2009).
The slow optical decay may be also due to the presence of a fairly
bright host galaxy.
Further optical observations are encouraged.