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Department of Physics

# Staff profile

## Publication details for Prof David Alexander

Chen(陳建廷), Chien-Ting J., Hickox, Ryan C., Goulding, Andrew D., Stern, Daniel, Assef, Roberto, Kochanek, Christopher S., Brown, Michael J. I., Harrison, Chris M., Hainline, Kevin N., Alberts, Stacey, Alexander, David M., Brodwin, Mark, Moro, Agnese Del, Forman, William R., Gorjian, Varoujan, Jones, Christine, Murray, Stephen S., Pope, Alexandra & Rovilos, Emmanouel (2017). The X-ray and mid-infrared luminosities in luminous type 1 quasars. The Astrophysical Journal 837(2): 145.

### Abstract

Several recent studies have reported different intrinsic correlations between the active galactic nucleus (AGN) mid-IR luminosity (${L}_{\mathrm{MIR}}$) and the rest-frame 2–10 keV luminosity (L X) for luminous quasars. To understand the origin of the difference in the observed ${L}{{\rm{X}}}\mbox{--}{L}{\mathrm{MIR}}$ 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 ${L}{{\rm{X}}}\mbox{--}{L}{\mathrm{MIR}}$ 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 ${L}{{\rm{X}}}\mbox{--}{L}{\mathrm{MIR}}$ 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 $6\,\mu {\rm{m}}$, or ${L}_{6\mu {\rm{m}}}$) of our sample of type 1 AGNs follow a bilinear relation in the log–log plane: $\mathrm{log}{L}_{{\rm{X}}}=(0.84\pm 0.03)\times \mathrm{log}{L}_{6\mu {\rm{m}}}/{10}^{45}$ erg s−1 + (44.60 ± 0.01) for ${L}_{6\mu {\rm{m}}}\lt {10}^{44.79}$ erg s−1, and $\mathrm{log}{L}_{{\rm{X}}}=(0.40\pm 0.03)\times \mathrm{log}{L}_{6\mu {\rm{m}}}/{10}^{45}$ erg s−1 + (44.51 ± 0.01) for ${L}_{6\mu {\rm{m}}}\,\geqslant {10}^{44.79}$ erg s−1. This suggests that the luminous type 1 quasars have a shallower ${L}{{\rm{X}}}\mbox{--}{L}{6\mu {\rm{m}}}$ correlation than the approximately linear relations found in local Seyfert galaxies. This result is consistent with previous studies reporting a luminosity-dependent ${L}{{\rm{X}}}\mbox{--}{L}{\mathrm{MIR}}$ relation and implies that assuming a linear ${L}{{\rm{X}}}\mbox{--}{L}{6\mu {\rm{m}}}$ relation to infer the neutral gas column density for X-ray absorption might overestimate the column densities in luminous quasars.