A Sample of He <scp>II</scp> <i>λ</i> 4686 “Changing-look” Quasars

Abstract We present the first systematic search for “changing-look” (“CL”) behavior in the broad He ii λ 4686 emission line in quasars, utilizing repeated spectroscopy from the Sloan Digital Sky Survey. The He ii line, originating from high-ionization gas and powered by extreme-ultraviolet photons, serves as a sensitive tracer of changes in the ionizing continuum. After applying strict spectral selection criteria and visual inspection to a parent sample of over 9000 quasars with multiepoch spectra, we identify a sample of 34 He ii “CL” quasars that show a significant appearance or disappearance of the broad He ii λ 4686 line. Compared with previously known H β “CL” quasars, the He ii “CL” sample exhibits similarly strong continuum variability and broad-line flux changes, yet shows a preference for higher Eddington ratios and lower host-galaxy contamination. These results highlight the value of the He ii line in studying the central variable engines of active galactic nuclei and uncovering a more complete census of extreme quasar variability. A comparison with H β “CL” further underscores the profound selection biases inherent in “CL” studies, especially those associated with line strength, host-galaxy contamination, and spectral signal-to-noise ratio.