BAT AGN Spectroscopic Survey – XII. The relation between coronal properties of active galactic nuclei and the Eddington ratio

Abstract

The bulk of the X-ray emission in Active Galactic Nuclei (AGN) is produced\nvery close to the accreting supermassive black hole (SMBH), in a corona of hot\nelectrons which up scatters optical and ultraviolet photons from the accretion\nflow. The cutoff energy ($E_{\\rm C}$) of the primary X-ray continuum emission\ncarries important information on the physical characteristics of the X-ray\nemitting plasma, but little is currently known about its potential relation\nwith the properties of accreting SMBHs. Using the largest broad-band (0.3-150\nkeV) X-ray spectroscopic study available to date, we investigate how the corona\nis related to the AGN luminosity, black hole mass and Eddington ratio\n($\\lambda_{\\rm Edd}$). Assuming a slab corona the median values of the\ntemperature and optical depth of the Comptonizing plasma are $kT_{\\rm e}=105\n\\pm 18$ keV and $\\tau=0.25\\pm0.06$, respectively. When we properly account for\nthe large number of $E_{\\rm C}$ lower limits, we find a statistically\nsignificant dependence of the cutoff energy on the Eddington ratio. In\nparticular, objects with $ \\lambda_{\\rm Edd}>0.1$ have a significantly lower\nmedian cutoff energy ($E_{\\rm C}=160\\pm41$ keV) than those with $\\lambda_{\\rm\nEdd}\\leq 0.1$ ($E_{\\rm C}=370\\pm51$ keV). This is consistent with the idea that\nradiatively compact coronae are also cooler, because they tend to avoid the\nregion in the temperature-compactness parameter space where runaway pair\nproduction would dominate. We show that this behaviour could also\nstraightforwardly explain the suggested positive correlation between the photon\nindex ($\\Gamma$) and the Eddington ratio, being able to reproduce the observed\nslope of the $\\Gamma-\\lambda_{\\rm Edd}$ trend.\n