The Subaru/XMM-Newton deep survey (SXDS).: V.: Optically faint variable object survey

We present our survey for optically faint variable objects using multiepoch ( 8-10 epochs over 2-4 years) i'- band imaging data obtained with Subaru Suprime-Cam over 0.918 deg(2) in the Subaru/XMM-Newton Deep Field (SXDF). We found 1040 optically variable objects by image subtraction for all the combinations of images at different epochs. This is the first statistical sample of variable objects at depths achieved with 8-10 m class telescopes or the Hubble Space Telescope. The detection limit for variable components is i'(vari) similar to 25.5 mag. These variable objects were classified into variable stars, supernovae ( SNe), and active galactic nuclei ( AGNs), based on the optical morphologies, magnitudes, colors, and optical-mid- infrared colors of the host objects, spatial offsets of variable components from the host objects, and light curves. Detection completeness was examined by simulating light curves for periodic and irregular variability. We detected optical variability for 36% +/- 2% ( 51% +/- 3% for a bright samplewith i' < 24.4mag) of X- ray sources in the field. Number densities of variable objects as functions of time intervals Delta t and variable component magnitudes i'vari are obtained. Number densities of variable stars, SNe, and AGNs are 120, 489, and 579 objects deg(-2), respectively. Bimodal distributions of variable stars in the color-magnitude diagrams indicate that the variable star sample consists of bright ( V similar to 22 mag) blue variable stars of the halo population and faint ( V similar to 23.5 mag) red variable stars of the disk population. There are a few candidates of RR Lyrae providing a possible number density of similar to 10(-2) kpc(-3) at a distance of > 150 kpc from the Galactic center.