Mon. Not. R. Astron. Soc., 504, 5556-5574 (2021/July-2)
What controls the UV-to-X-ray continuum shape in quasars?
TIMLIN III J.D., BRANDT W.N. and LAOR A.
Abstract (from CDS):
We present an investigation of the interdependence of the optical-to-X-ray spectral slope (αox), the He II equivalent-width (EW), and the monochromatic luminosity at 2500 Å (L2500). The values of αox and He II EW are indicators of the strength/shape of the quasar ionizing continuum, from the ultraviolet (UV; 1500-2500 Å), through the extreme ultraviolet (EUV; 300-50 Å), to the X-ray (2 keV) regime. For this investigation, we measure the He II EW of 206 radioquiet quasars devoid of broad absorption lines that have high-quality spectral observations of the UV and 2 keV X-rays. The sample spans wide redshift (≃0.13-3.5) and luminosity (log(L2500) ≃ 29.2-32.5 erg s–1 Hz–1) ranges. We recover the well-known αox-L2500 and He II EW-L2500 anticorrelations, and we find a similarly strong correlation between αox and He II EW, and thus the overall spectral shape from the UV, through the EUV, to the X-ray regime is largely set by luminosity. A significant αox- He II EW correlation remains after removing the contribution of L2500 from each quantity, and thus the emission in the EUV and the X-rays are also directly tied. This set of relations is surprising, since the UV, EUV, and X-ray emission are expected to be formed in three physically distinct regions. Our results indicate the presence of a redshift-independent physical mechanism that couples the continuum emission from these three different regions, and thus controls the overall continuum shape from the UV to the X-ray regime.
© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society
galaxies: active - quasars: emission lines - quasars: general - X-rays: general
VizieR on-line data:
<Available at CDS (J/MNRAS/504/5556): tablea1.dat tableb1.dat>
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