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| Main Authors: | , |
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| Format: | Preprint |
| Published: |
2025
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| Online Access: | https://arxiv.org/abs/2510.06753 |
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| _version_ | 1866916997139267584 |
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| author | Lu, Wentao Wang, Jun-Xian |
| author_facet | Lu, Wentao Wang, Jun-Xian |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_06753 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | A Sample of HeII $λ$4686 "Changing-Look" Quasars Lu, Wentao Wang, Jun-Xian Astrophysics of Galaxies 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. |
| title | A Sample of HeII $λ$4686 "Changing-Look" Quasars |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2510.06753 |