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"Changing-look" quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of just a few years. The origin of this phenomenon is debated, but is likely to reflect some change in the accretion flow. In this talk, I will introduce our work on investigating the disk-corona systems in ten previously discovered changing-look quasars, with X-ray data from Chandra, XMM-Newton, and ROSAT, and optical spectroscopy from SDSS, MMT, Magellan, HET, and ARC 3.5m Telescope. We measure their optical/UV-X-ray spectral indices and Eddington ratios at multiple epochs, and find possible similarities in spectral indices below 1% Eddington ratio between our measurements and simulated results based on the behavior of X-ray binaries. We further investigate the Eddington ratios of changing-look quasars before and after their spectral type changes, and find that changing-look quasars cross the 1% Eddington ratio boundary when their broad emission lines disappear/emerge. This is consistent with the disk-wind model as the origin of broad emission lines.
Xiangyu Jin was an undergrad at Nanjing University. She then went to McGill University for her master degree. She is now pursuing her PhD in astronomy at the University of Arizona. Her research interests include the formation and evolution of supermassive black holes, and time domain study of supermassive black holes.