Multiepoch sky surveys and the lifetime of quasars

We present a new method for measuring the episodic lifetime of quasars with current and future large-scale sky surveys. Future photometric observations of large samples of confirmed quasars can provide a direct measurement (or interesting lower limit) to the lifetime of an individual episode of quasar activity (t_Q) and potentially enable the study of postquasar host galaxies. Photometric observations of the quasars found by the Sloan Digital Sky Survey (SDSS) and Two-Degree Field Survey could, with a time baseline of 10 years, determine t_Q to within a factor of 2 if t _Q < 10⁵ yr or set a lower limit to the quasar lifetime. Multiple-epoch precise photometry with the proposed Large Synoptic Survey Telescope could test more complex models for quasar variability and mean quasar luminosity evolution. These observations could also constrain the rate that tidal disruptions of single stars produce quasar luminosities. It is possible to reverse the order of this investigation; previous-epoch plate material, such as the Digitized Sky Survey, can be used to determine if any of the SDSS quasars had not yet turned on at the time of these prior observations. Measurements of the entire SDSS quasar sample over the ∼50 yr baseline provided by these plates potentially can be used to estimate t_Q to within a factor of 2 if t_Q < 10^5.5 yr, provided quasar variability can be accurately characterized and the detection efficiency and photometric calibration of the plate material can be well determined. These measurements of t_Q will have comparable quality to existing, more indirect estimates of the quasar lifetime. Analysis of the 3814 quasars in the SDSS Early Data Release finds that t_Q must be larger than approximately 20,000 yr.