Presentation #106A.04 in the session Active Galactic Nuclei I.
Virtually unknown a decade ago, dozens of ‘changing-look quasars’ (CLQs) have recently been found, where UV continuum and broad emission lines drop (or rise) dramatically. CLQ transitions have been attributed to tidal disruption events, significant changes in intrinsic absorption or in accretion rate, but all these hypotheses suffer theoretical or empirical challenges. To study the transitions in more detail at quasar luminosities, we present a pilot study of a sample of SDSS quasars with archival X-ray observations that we have confirmed as CLQs from strong changes in their optical continuum and Hβ emission. We have obtained Chandra X-ray observations for 5 CLQs, as well as optical spectroscopic observations and VLA radio imaging in the dim state, previous (XMM-Newton or Chandra) X-ray data, and SDSS optical spectra, in the bright state. We characterize CLQ changes in nuclear X-ray luminosity, intrinsic absorption, and accretion rate, testing several competing models, including analogies to the different accretion states of X-ray binaries (XRBs). Dim-state CLQs allow new tests of the ‘fundamental plane’ that connects radio and X-ray emission from black hole at low accretion rates across ~8 orders of magnitude in mass from XRBs to supermassive black holes. This work was supported by Chandra grants GO8-19089X, GO9-20086X, and GO0-21084X.