Astronomy Colloquium

Quasars are known to be an optically variable population but the physical mechanisms behind this remain unclear. Variability provides information that is complementary to the studies of their spectral energy distributions, and can thus lead to new insights into the physics of AGN. The availability of large collections of astronomical time series, e.g., from CRTS, PTF, and PAN-STARRs, is now supporting systematic analyses of both the normal stochastic variability of quasars, as well as of the rare and extreme behaviors.
It is still unclear, however, whether these represent the tail of general quasar variability or a different class of phenomena but both have implications for quasar models.  In this talk I will review the different extreme categories we have identified so far: close (subparsec) supermassive black hole binaries, flaring sources suggestive of explosive stellar-related activity in the accretion disk, microlensed sources, and dramatic spectral variability coupled with strong, persistent changes in luminosity. I will also consider upcoming possibilities for further detection, characterization and modeling of these objects.