High-redshift Extreme Variability Quasars from Sloan Digital Sky Survey Multiepoch Spectroscopy

• Published in: The Astrophysical journal 2020-12, Vol.905 (1), p.52
• Main Authors: Guo, Hengxiao, Peng, Jiacheng, Zhang, Kaiwen, Burke, Colin J., Liu, Xin, Sun, Mouyuan, Wang, Shu, Kong, Minzhi, Sheng, Zhenfeng, Wang, Tinggui, He, Zhicheng, Gu, Minfeng
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Astrophysics; Black hole physics; Deposition; Emission lines; Line spectra; Luminosity; Quasars; Red shift; Sky surveys (astronomy); Spectroscopy; Spectrum analysis; Time domain astronomy; Variability
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abc2ce

We perform a systematic search for high-redshift ( 1.5) extreme variability quasars (EVQs) using repeat spectra from the Sixteenth Data Release of the Sloan Digital Sky Survey, which provides a baseline spanning up to ∼18 yr in the observed frame. We compile a sample of 348 EVQs with a maximum continuum variability at rest frame 1450 of more than 100% (i.e., δV (Max − Min)/Mean > 1). The EVQs show a range of emission-line variability, including 23 where at least one line in our redshift range disappears below detectability, which can then be seen as analogous to low-redshift changing-look quasars (CLQs). Importantly, spurious CLQs caused by problematic SDSS spectral flux calibration, e.g., fiber-drop issue, have been rejected. The similar properties (e.g., continuum/line, difference-composite spectra and Eddington ratio) of normal EVQs and CLQs imply that they are basically the same physical population with analogous intrinsic variability mechanisms, as a tail of a continuous distribution of normal quasar properties. In addition, we find no reliable evidence ( 1 ) to support that CLQs are a subset of EVQs with less efficient accretion. Finally, we also confirm the antibreathing of C iv (i.e., the line width increases as luminosity increases) in EVQs and find that in addition to the ∼0.4 dex systematic uncertainty in single-epoch C iv virial black hole mass estimates, an extra scatter of ∼0.3 dex will be introduced by extreme variability.