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The Astrophysical Journal


Department of Physics and Astronomy


Several recent studies have reported different intrinsic correlations between the AGN mid-IR luminosity (LMIR) and the rest-frame 2-10 keV luminosity (LX) for luminous quasars. To understand the origin of the difference in the observed LX−LMIR relations, we study a sample of 3,247 spectroscopically confirmed type 1 AGNs collected from Bo\"{o}tes, XMM-COSMOS, XMM-XXL-North, and the SDSS quasars in the Swift/XRT footprint spanning over four orders of magnitude in luminosity. We carefully examine how different observational constraints impact the observed LX−LMIR relations, including the inclusion of X-ray non-detected 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 LX−LMIR 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 6μm, or L6μm) of our sample of type 1 AGNs follow a bilinear relation in the log-log plane: logLX=(0.84±0.03)×logL6μm/1045ergs−1+(44.60±0.01) for L6μm<1044.79ergs−1, and logLX=(0.40±0.03)×logL6μm/1045ergs−1+(44.51±0.01) for L6μm≥1044.79ergs−1. This suggests that the luminous type 1 quasars have a shallower LX−LMIR correlation than the approximately linear relations found in local Seyfert galaxies. This result is consistent with previous studies reporting a luminosity-dependent LX−LMIR relation, and implies that assuming a linear LX−LMIR relation to infer the neutral gas column density for X-ray absorption might overestimate the column densities in luminous quasars