Constraints on the duty cycle and duration of accretion episodes in active galactic nuclei (AGNs) are vital for establishing how most AGNs are fueled, which is essential for a complete picture of black hole/galaxy co-evolution. Perhaps the best handle we have on these activity parameters is provided by AGNs that have displayed dramatic, persistent changes in their bolometric luminosities. Given that X-ray emission is directly linked to black-hole accretion, X-ray source catalogs should provide a straightforward means of identifying AGNs that have undergone dramatic changes in their accretion states. However, it appears that such events are very rare, so observations separated in time by many years are needed to maximize discovery rates. We have cross-correlated the Einstein IPC Two-Sigma Catalog with the Chandra Source Catalog to identify a sample of soft X-ray sources that varied by factors of at least 10 over timescales of 20 or more years. Most of the extremely variable sources revealed via this technique were dimmer in the Chandra era, owing to the lower sensitivity and modest exposures associated with Einstein. Optical spectra from the Sloan Digital Sky Survey indicate that many of the variable sources are radio-quiet AGNs with broad emission lines — not \"changing-look\" objects that have more subtle AGN signatures in their spectra. We present long-term X-ray light curves for the sources by combining the Einstein and CSC fluxes with those obtained from serendipitous pointed observations by ROSAT.
"},{"label":"Degree","value":["Bachelor of Arts with Departmental Honors"]},{"label":"Author-supplied Keywords","value":["X-ray variability","Active galactic nuclei accretion","Changing-look active galactic nuclei"]},{"label":"Administrative Note","value":"Drupal Node ID: 24035