GCN Circular 8999

A. de Ugarte Postigo (ESO), J. Gorosabel, A. Sota,
A.J. Castro-Tirado (IAA-CSIC), S. McBreen (Univ.
College Dublin) and M.R. Zapatero-Osorio (IAC)
report on behalf of a larger collaboration:

We continued the observations of the afterglow (Chornock
et al. GCNC 8979) of GRB 090313 (Mao et al. GCNC 8980)
from the 1.5m OSN telescope at Sierra Nevada Observatory
(Granada, Spain) in I-band, the 0.8m IAC80 telescope at
Teide Observatory (Tenerife, Spain) in R-band and the
2.5m NOT telescope in nIR bands.

The observations show how the light curve maintains its
brightness until at least 0.8341 days after the burst,
where our last image from OSN shows the afterglow at
I=17.9+/-0.2. This is still stable as compared to the
report of Perley et al. (GCNC 8985) 0.12 days after
the burst and our previous report (de Ugarte Postigo
et al. GCNC 8992) 0.56 days after the burst. We note
that our new magnitude is somehow brighter than the one
reported by Perley et al. at a similar epoch (GCNC
8997). This could be due to a calibration issue. We are
using USNO-B1.0 stars as reference.

From the IAC80 we observe the burst during the plateau
0.6482 days after the burst at a magnitude of 19.1+/-0.1.

At NOT telescope we clearly detect the bright afterglow in
J and Ks bands with NOTCAM 0.7989 days after the burst at a
magnitude of Ks=15.2+/-0.3. We used 2MASS for the calibration,
which is the main source of uncertainty in this value.

Observations continued the following night, where we see a
strong drop in the magnitude of the afterglow, which can be
detected at I=20.2 +/- 0.4, 1.5547 days after the burst,
indicating the presence of a break in the light curve.