The subarcsecond mid-infrared view of local active galactic nuclei – II. The mid-infrared–X-ray correlation

Abstract

We present an updated mid-infrared (MIR) versus X-ray correlation for the\nlocal active galactic nuclei (AGN) population based on the high angular\nresolution 12 and 18um continuum fluxes from the AGN subarcsecond MIR atlas and\n2-10 keV and 14-195 keV data collected from the literature. We isolate a sample\nof 152 objects with reliable AGN nature and multi-epoch X-ray data and minimal\nMIR contribution from star formation. Although the sample is not homogeneous or\ncomplete, we show that our results are unlikely to be affected by biases. The\nMIR--X-ray correlation is nearly linear and within a factor of two independent\nof the AGN type and the wavebands used. The observed scatter is <0.4 dex. A\npossible flattening of the correlation slope at the highest luminosities probed\n(~ 10^45 erg/s) is indicated but not significant. Unobscured objects have, on\naverage, an MIR--X-ray ratio that is only <= 0.15 dex higher than that of\nobscured objects. Objects with intermediate X-ray column densities (22 < log\nN_H < 23) actually show the highest MIR--X-ray ratio on average. Radio-loud\nobjects show a higher mean MIR--X-ray ratio at low luminosities, while the\nratio is lower than average at high luminosities. This may be explained by\nsynchrotron emission from the jet contributing to the MIR at low-luminosities\nand additional X-ray emission at high luminosities. True Seyfert 2 candidates\nand double AGN do not show any deviation from the general behaviour. Finally,\nwe show that the MIR--X-ray correlation can be used to verify the AGN nature of\nuncertain objects. Specifically, we give equations that allow to determine the\nintrinsic 2-10 keV luminosities and column densities for objects with complex\nX-ray properties to within 0.34 dex. These techniques are applied to the\nuncertain objects of the remaining AGN MIR atlas, demonstrating the usefulness\nof the MIR--X-ray correlation as an empirical tool.\n