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
Not found
 
 
, 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
Not found
 
 
), 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
Not found
 
 
, Gal-Yam
et al. GCN 3690
Not found
 
 
, 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.