TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3681
SUBJECT: GRB 050724: Optical Variability in Nearby Galaxy
DATE:    05/07/26 07:50:06 GMT
FROM:    S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>

A. Gal-Yam, S. B. Cenko (Caltech), E. Berger (Carnegie), W. Krzeminski
(LCO) and B. Lee (Toronto) report on behalf of the Caltech-NRAO-Carnegie
collaboration:

We have obtained three epochs of optical imaging of the field of GRB
050724 (Covino et al., GCN 3665) with the RetroCam CCD Camera on the
1.0-m Swope telescope at Las Campanas Observatory.  Each epoch consisted
of 2 x 15 min exposures in I-band, with the last epoch also containing one
additional ten minute exposure.

We subtracted the coadded sum image obtained during the second night of
observations (epoch 3; July 26.05 UT) from the coadded sum images taken
during the first night (epoch 1, July 25.02 UT; epoch 2, July 25.11 UT ) using
the CPM (Gal-Yam et al. 2004) and ISIS (Alard et al. 1999) codes. We
clearly detect a variable point source superposed on galaxy "D" of
Bloom et al. (GCN 3672).

The variable source is offset from the galaxy core (see figure at
http://www.its.caltech.edu/~cenko/grb050724/grb050724.jpg; the green
circle is centered on the galaxy core from epoch 3, the red is centered
on the OT).  Furthermore, the light distribution in epoch 1 is markedly
different from that in epoch 3, due to the effect of the OT light,
disfavoring the possibility that the optical variability results from
Galaxy D being an AGN.

Calibrating the OT magnitudes against "I2" magnitudes of four nearby stars
from the USNO B1 catalog, we find that the OT brightened by 0.6
magnitudes between epochs 1 and 2 (I=21.25 at epoch 1, 12 hours after
the burst; I=20.65 at epoch 2, 14.3 hours after the GRB). We note that
such behavior is rare among optical afterglows.