AGN X-ray coronae at high luminosity: A broadband study of RBS 229 and PG 1407+265

Résumé fourni par la source

The X-ray emission of active galactic nuclei (AGNs) is commonly interpreted as arising from Comptonisation in a hot corona located above the accretion disc. In many sources, a prominent soft X-ray excess is also observed, often explained within the two-corona scenario, where a warm Comptonising layer produces the soft excess, while a hotter corona generates the hard X-ray continuum. However, the physical properties of these coronal components remain poorly constrained in very luminous quasars. We investigate the structure and physical properties of the X-ray-emitting coronae in two luminous radio-quiet quasars, PG 1407+265 (z=0.94) and RBS 229 (z=0.334), and test whether the two-corona scenario provides a consistent description of their broadband spectra. We performed a broadband spectral analysis combining EPIC and Optical Monitor data with simultaneous observations and long-term monitoring over months to years. The spectra were modelled using Comptonisation models to characterise the properties of both the warm and hot coronae. XMM-Newton NuSTAR Swift Both quasars are X-ray-loud with respect to the standard α_̊m OX--L_̊m UV relation, with PG 1407+265 exhibiting the strongest deviation. Their broadband spectra are well reproduced by a two-corona configuration, consisting of a warm, optically thick Comptonising component that accounts for the soft excess and a hot corona responsible for the hard X-ray continuum. Both sources show high energy cut-offs in the range sim70-80 keV. RBS 229 exhibits moderate reflection and a weak neutral Fe Kα line, whereas no significant reflection features were detected in PG 1407+265. The soft excess is present in both quasars, but displays a markedly different behaviour for each: the warm-corona parameters in RBS 229 remain consistent across epochs, while PG 1407+265 displays strong variability, producing an extreme soft-excess episode extending up to sim3 keV in the rest frame. These results provide new constraints on the properties of AGN coronae in the high-luminosity regime, which remains relatively poorly explored in observations. Both quasars show high energy cut-offs that are broadly consistent with the trends observed in terms of luminosity and accretion rates among AGN samples and safely below the runaway pair-production limit. No clear dependence of the warm-corona properties on the luminosity or the accretion rate was found, as both quasars show similar kT, τ, and soft-excess strengths despite their different regimes, with variations seen mainly in terms of the spectral slope and variability. The strong variability of the warm component in PG 1407+265, compared to the stability of RBS 229, supports a scenario in which the warm corona responds to accretion-flow changes, while remaining within a narrow range of physical conditions.

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