The X-Ray and Mid-infrared Luminosities in Luminous Type 1 Quasars

Several recent studies have reported different intrinsic correlations between the active galactic nucleus (AGN) mid-IR luminosity (L_MIR) and the rest-frame 2-10 keV luminosity (L_X) for luminous quasars. To understand the origin of the difference in the observed L_X-L_MIR relations, we study a sample of 3247 spectroscopically confirmed type 1 AGNs collected from Boötes, XMM-COSMOS, XMM-XXL-North, and the Sloan Digital Sky Survey quasars in the Swift/XRT footprint spanning over four orders of magnitude in luminosity. We carefully examine how different observational constraints impact the observed L_X-L_MIR relations, including the inclusion of X-ray-nondetected objects, possible X-ray absorption in type 1 AGNs, X-ray flux limits, and star formation contamination. We find that the primary factor driving the different L_X-L_MIR relations reported in the literature is the X-ray flux limits for different studies. When taking these effects into account, we find that the X-ray luminosity and mid-IR luminosity (measured at rest-frame 6 μm, or L₆ μm) of our sample of type 1 AGNs follow a bilinear relation in the log-log plane: log L_X = (0.84 ± 0.03) × log L₆ μm 10⁴⁵ erg s^-1 + = (44.60 ± 0.01) for L₆ μ <10^44.79 erg s^-1, and log L_X = (0.40±0.03) × log L₆ μm/10⁴⁵ + erg s^-1 +(44.51 ± 0.01) for L₆ μm ≥ 10^44.79 erg s^-1. This suggests that the luminous type 1 quasars have a shallower L_X-L₆ μm correlation than the approximately linear relations found in local Seyfert galaxies. This result is consistent with previous studies reporting a luminosity-dependent L_X-L_MIR relation and implies that assuming a linear L_X-L₆ μm relation to infer the neutral gas column density for X-ray absorption might overestimate the column densities in luminous quasars.