GCN Circular 3694
B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS consortium, report: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 050724 (GCN 3665, Covino et al.) at two separate epochs. The two epochs had mid-exposure times of 2005-07-25 01:58 UT (13.4 hours post-burst) and 2005-07-27 03:21 UT (62.8 hours post-burst). Total summed exposure times at each epoch amounted to 36 minutes in I and 30 minutes in J, resulting in approximate limiting magnitudes of I>22 and J>20. In order to search for variability in the host galaxy of GRB 050724 ("source D" from Bloom et al. GCN 3672), the ISIS image subtraction routine was used (Alard et al. 1999). When the second epoch images are subtracted from the first epoch images, the residual light of the afterglow is clearly detected in both the I and J subtracted frames (in agreement with both GCN 3681, Gal-Yam et al. GCN 3690, D'Avanzo et al.). The centroid of this residual is ~0.2" west and ~0.5" south of the centroid of the host galaxy. Preliminary differential photometry of the host galaxy yields a decay of 0.19 +/- 0.03 magnitudes in I and 0.17 +/- 0.05 magnitudes in J between the two imaging epochs. Assuming the afterglow no longer contributes significantly to the brightness of the host galaxy at the second epoch, then the magnitude of the host galaxy is I = 18.78 +/- 0.27 and J = 16.94 +/- 0.09. Unfortunately, imaging was done under non-photometric conditions so no Landolt or Persson standard stars are available with which to determine the offset between instrumental and apparent magnitude. Therefore, the above values are determined using "on-chip" standards (USNO-B1 stars in the optical and 2MASS stars in the IR) and the error is strongly dominated by the uncertainty in the offset derived from these stars. Given the above values for the magnitude of the host and its dimming, the afterglow of GRB 050724 is determined to have a magnitude of I = 20.58 +/- 0.32 and J = 18.87 +/- 0.32 at 13.4 hours post-burst.