Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. V. Dramatic Variability in High-ionization Broad Emission Lines

Abstract We present a systematic search for changing-look (CL) quasars at high redshift z &gt; 0.9 by crossmatching the spectroscopic data sets from the Dark Energy Spectroscopic Instrument Data Release 1 and Sloan Digital Sky Survey Data Release 18. We identify 97 CL quasars showing significant variability in high-ionization broad emission lines, including 45 turn-on and 52 turn-off events, corresponding to a detection rate of ∼0.042%. The low rate relative to low-ionization CL quasar searches is likely due to selection and physical effects in high-ionization lines. Based on the CL quasar sample, we find that CL quasars generally exhibit lower accretion rates compared to typical quasars, with average Eddington ratios of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Edd</mml:mi> </mml:mrow> </mml:msub> </mml:math> ∼ −1.14 in the bright state and ∼−1.39 in the dim state, compared to ∼−0.65 for typical quasars. Furthermore, while high-ionization lines in CL quasars follow the Baldwin effect on a population level, some sources can display inverse Baldwin trends. In addition, we find a positive correlation between the variability in high-ionization lines (e.g., Mg ii , C iii ]) and the change in bolometric luminosity. We also estimate a characteristic rest-frame timescale of ∼3 yr for CL transitions, with no significant difference between turn-on and turn-off cases. Taken as a whole, these findings support an accretion-driven origin of the CL phenomenon, and provide new insights into the variability of high-ionization emission lines.