Wavelength dependences of the optical/UV and X-ray luminosity correlations of quasars

Ward, Martin; Done, Christine; Jin, Chichuan; Lusso, Elisabeta; Middei, Riccardo

Abstract

The interband correlations between optical/ultraviolet (UV) and X-ray luminosities of active galactic nuclei (AGNs) are important
for understanding the disc–coronal connection, as well as using AGN as standard candles for cosmology. It is conventional to
measure the X-ray luminosity at rest-frame 2 keV and compare to the UV luminosity at the rest-frame 2500 Å, but the wavelength
dependence was never well explored. In this work, we adopt a well-defined sample of 1169 unobscured quasars in the redshift
range 0.13–4.51, and apply the direct-correlation method to explore how the correlation with the 2 keV luminosity changes at
different optical/UV wavelengths, from 1280 to 5550 Å where the spectral quality is high. We find that the luminosity at all UV
continuum wavelengths correlates with the X-ray luminosity similarly to that at 2500 Å, and that these correlations are better
than at the optical wavelengths. Strong self-correlation is also found in the broad-band optical/UV continuum, supporting the
scenario that it is dominated by the disc emission. Correlations of various emission lines are also investigated (e.g. C IV, C III],
Mg II, Hβ, and [O III]λλ4959/5007), including the Baldwin effect and correlations involving linewidths. We find the forms of
these line correlations are different, and they are also different from their underlying continua, suggesting various complexities in
the line-generation process. We discuss these results in the disc-wind scenario. Our study confirms that the rest-frame 2500 Å is a
good wavelength to represent the optical/UV continual properties of quasars, and shows the advantages of the direct-correlation
method.

Citation

Ward, M., Done, C., Jin, C., Lusso, E., & Middei, R. (2024). Wavelength dependences of the optical/UV and X-ray luminosity correlations of quasars. Monthly Notices of the Royal Astronomical Society, 527(1), 356–373. https://doi.org/10.1093/mnras/stad3193

• Journal Article Type: Article
• Acceptance Date: Oct 14, 2023
• Online Publication Date: Oct 23, 2023
• Publication Date: 2024-01
• Deposit Date: Jan 29, 2024
• Publicly Available Date: Jan 29, 2024
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 527
• Issue: 1
• Pages: 356–373
• DOI: https://doi.org/10.1093/mnras/stad3193
• Public URL: https://durham-repository.worktribe.com/output/2186136