Supermassive Black Hole Winds in X-rays: SUBWAYS V. Properties of hot coronae in quasars at intermediate-redshift

We present the X-ray analysis of coronal properties in a statistically representative sample of 23 mostly radio-quiet active galactic nuclei (AGN) from the SUpermassive Black holes Winds in X-rAYs campaign (SUBWAYS), focusing on quasars at redshifts 0.1 < z < 0.4 and bolometric luminosities 2 × 1044 < Lbol(erg/s) < 2 × 1046. The main aim of this work is to investigate the properties of the hot corona through the study of the hard X-ray band emission, including a proper treatment of the soft X-ray band. High-quality X-ray spectra from XMM-Newton, complemented by NuSTAR data extending up to 30–40 keV in the rest frame, are available for this sample. The soft X-ray band (0.3–2 keV) spectrum is best fit by a warm corona model with a median temperature of ∼0.40 keV, and an optical depth in the range τ ∼ 20 − 40, consistent with previous results on lower-luminosity sources. The hard X-ray band is well described using a hot corona model, with a median high-energy cutoff of ∼87 keV; 11 out of 23 of the sources have well constrained values at the lower end of the distribution of typical values found in Seyfert galaxies (∼100 − 200 keV). The derived median value of the optical depth (τ ∼ 1 − 5) suggests the presence of a moderately optically thick corona. By combining the SUBWAYS results with literature samples at low and high redshifts, we assembled the largest sample to date of AGN with Ecut and accretion parameter measurements, finding a significant anticorrelation of Ecut with both λEdd and Lbol with the median Ecut decreasing from ∼250 − 300 keV at low accretion rates and luminosities to ∼90 − 100 keV at high accretion rates and luminosities; this is consistent with increased coronal cooling, possibly driven by pair production. These results favor cooler, optically thicker coronae in luminous AGN compared to those in lower-luminosity Seyfert galaxies.