A Sample of HeII $λ$4686 "Changing-Look" Quasars

TL;DR

This paper presents the first systematic search for changing-look behavior in the broad He II λ4686 line using multi-epoch SDSS spectroscopy, identifying 34 He II CL quasars. The authors develop a robust spectral fitting pipeline with host subtraction, Fe II modeling, and a nine-Gaussian decomposition of key lines, anchored by [O III] flux for flux calibration and validated with Monte Carlo uncertainties. They find that He II CL quasars exhibit strong continuum variability and broad He II flux changes comparable to or larger than Hβ CL quasars, and tend to have higher Eddington ratios with lower host contamination than Hβ CL counterparts, suggesting selection effects and intrinsic differences in variability drivers. The intrinsic occurrence rate is about 0.5%, but detection is strongly influenced by S/N, EW of He II, Fe II blending, and redshift, with minimal overlap with Hβ CL quasars due to traditional CL definitions. Overall, He II λ4686 offers a valuable, complementary window into extreme AGN variability and highlights the need to account for line-specific selection biases to obtain a complete census of changing-look phenomena.

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 (SDSS). 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 9,000 quasars with multi-epoch spectra, we identify a sample of 34 He ii "changing-look" 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 He ii line in studying the central variable engines of AGNs and uncovering a more complete census of extreme quasar variability. A comparison with H$β$ "CL" further underscores the profound selection biases inherent in "changing-look" studies, especially those associated with line strength, host-galaxy contamination, and spectral signal-to-noise ratio.

Figures (14)

• Figure 1: Two example artificial spectra (upper and lower panels) generated by adding random Gaussian noise to a real SDSS quasar spectrum with strong Fe ii emission (SDSS J090112.04+011549.5). In the left panels, the host-subtracted spectra are shown in blue, with the best-fit power-law continuum and power-law plus Fe ii components over-plotted in green and yellow, respectively. The red lines indicate the residual spectra after subtracting the model continuum. Vertical dotted lines mark the position of the He ii 4686 emission line. In the right panels, the residual spectra are re-plotted alongside the corresponding best-fit He ii and H$\beta$ emission line models. This figure illustrates the degeneracy between Fe ii and He ii 4686, and highlights how inaccurate Fe ii modeling can lead to spurious broad He ii features in the residual spectrum.
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