{
  "bibcode": "2005GCN..3694....1C",
  "body": "B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS\nconsortium, report:\n\nUsing the ANDICAM instrument on the 1.3m telescope at CTIO, we\nobtained optical/IR imaging of the error region of GRB 050724\n(GCN 3665, Covino et al.) at two separate epochs.\nThe two epochs had mid-exposure times of 2005-07-25 01:58 UT\n(13.4 hours post-burst) and 2005-07-27 03:21 UT (62.8 hours\npost-burst).  Total summed exposure times at each epoch\namounted to 36 minutes in I and 30 minutes in J, resulting in\napproximate limiting magnitudes of I>22 and J>20.\n\nIn order to search for variability in the host galaxy of GRB 050724\n(\"source D\" from Bloom et al. GCN 3672), the ISIS image subtraction\nroutine was used (Alard et al. 1999).  When the second epoch\nimages are subtracted from the first epoch images, the residual light\nof the afterglow is clearly detected in both the I and J\nsubtracted frames (in agreement with both GCN 3681, Gal-Yam\net al. GCN 3690, D'Avanzo et al.).  The centroid of this residual is ~0.2\"\nwest and ~0.5\" south of the centroid of the host galaxy.\n\nPreliminary differential photometry of the host galaxy yields a decay\nof 0.19 +/- 0.03 magnitudes in I and 0.17 +/- 0.05 magnitudes in J\nbetween the two imaging epochs.\n\nAssuming the afterglow no longer contributes significantly to the\nbrightness of the host galaxy at the second epoch, then the magnitude\nof the host galaxy is I = 18.78 +/- 0.27 and J = 16.94 +/- 0.09.\nUnfortunately, imaging was done under non-photometric conditions so\nno Landolt or Persson standard stars are available with which\nto determine the offset between instrumental and apparent\nmagnitude.  Therefore, the above values are determined using \"on-chip\"\nstandards (USNO-B1 stars in the optical and 2MASS stars in the IR) and the\nerror is strongly dominated by the uncertainty in the offset derived from\nthese stars.\n\nGiven the above values for the magnitude of the host and its\ndimming, the afterglow of GRB 050724 is determined to have a magnitude of\nI = 20.58 +/- 0.32 and J = 18.87 +/- 0.32 at 13.4 hours post-burst.",
  "circularId": 3694,
  "createdOn": 1122575053000,
  "email": "cobb@astro.yale.edu",
  "subject": "GRB 050724, SMARTS optical/IR afterglow observations",
  "submitter": "Bethany Cobb at Yale U  <cobb@astro.yale.edu>",
  "eventId": "GRB 050724"
}