A Dark Energy Camera Search for an Optical Counterpart to the First
  Advanced LIGO Gravitational Wave Event GW150914
  We report initial results of a deep search for an optical counterpart to the
gravitational wave event GW150914, the first trigger from the Advanced LIGO
gravitational wave detectors. We used the Dark Energy Camera (DECam) to image a
102 deg$^2$ area, corresponding to 38% of the initial trigger high-probability
sky region and to 11% of the revised high-probability region. We observed in i
and z bands at 4-5, 7, and 24 days after the trigger. The median $5\sigma$
point-source limiting magnitudes of our search images are i=22.5 and z=21.8
mag. We processed the images through a difference-imaging pipeline using
templates from pre-existing Dark Energy Survey data and publicly available
DECam data. Due to missing template observations and other losses, our
effective search area subtends 40 deg$^{2}$, corresponding to 12% total
probability in the initial map and 3% of the final map. In this area, we search
for objects that decline significantly between days 4-5 and day 7, and are
undetectable by day 24, finding none to typical magnitude limits of i=
21.5,21.1,20.1 for object colors (i-z)=1,0,-1, respectively. Our search
demonstrates the feasibility of a dedicated search program with DECam and bodes
well for future research in this emerging field.
