{
  "bibcode": "2005GCN..3681....1G",
  "body": "A. Gal-Yam, S. B. Cenko (Caltech), E. Berger (Carnegie), W. Krzeminski\n(LCO) and B. Lee (Toronto) report on behalf of the Caltech-NRAO-Carnegie\ncollaboration:\n\nWe have obtained three epochs of optical imaging of the field of GRB\n050724 (Covino et al., GCN 3665) with the RetroCam CCD Camera on the\n1.0-m Swope telescope at Las Campanas Observatory.  Each epoch consisted\nof 2 x 15 min exposures in I-band, with the last epoch also containing one\nadditional ten minute exposure.\n\nWe subtracted the coadded sum image obtained during the second night of\nobservations (epoch 3; July 26.05 UT) from the coadded sum images taken\nduring the first night (epoch 1, July 25.02 UT; epoch 2, July 25.11 UT ) using\nthe CPM (Gal-Yam et al. 2004) and ISIS (Alard et al. 1999) codes. We\nclearly detect a variable point source superposed on galaxy \"D\" of\nBloom et al. (GCN 3672).\n\nThe variable source is offset from the galaxy core (see figure at\nhttp://www.its.caltech.edu/~cenko/grb050724/grb050724.jpg; the green\ncircle is centered on the galaxy core from epoch 3, the red is centered\non the OT).  Furthermore, the light distribution in epoch 1 is markedly\ndifferent from that in epoch 3, due to the effect of the OT light,\ndisfavoring the possibility that the optical variability results from\nGalaxy D being an AGN.\n\nCalibrating the OT magnitudes against \"I2\" magnitudes of four nearby stars\nfrom the USNO B1 catalog, we find that the OT brightened by 0.6\nmagnitudes between epochs 1 and 2 (I=21.25 at epoch 1, 12 hours after\nthe burst; I=20.65 at epoch 2, 14.3 hours after the GRB). We note that\nsuch behavior is rare among optical afterglows.",
  "circularId": 3681,
  "createdOn": 1122364206000,
  "email": "cenko@srl.caltech.edu",
  "subject": "GRB 050724: Optical Variability in Nearby Galaxy",
  "submitter": "S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>",
  "eventId": "GRB 050724"
}