CAIXA: a Catalogue of AGN In the XMM-Newton Archive II. Multiwavelength
  correlations
  We presented CAIXA, a Catalogue of AGN in the XMM-Newton Archive, in a
companion paper. Here, a systematic search for correlations between the X-ray
spectral properties and the multiwavelength data was performed for the sources
in CAIXA. All the significant (>99.9% confidence level) correlations are
discussed along with their physical implications on current models of AGN. Two
main correlations are discussed in this paper: a) a very strong
anti-correlation between the FWHM of the H$\beta$ optical line and the ratio
between the soft and the hard X-ray luminosity. Although similar
anti-correlations between optical line width and X-ray spectral steepness have
already been discussed in the literature (see e.g., Laor et al. 1994, Boller et
al. 1996, Brandt et al. 1997), we consider the formulation we present in this
paper is more fundamental, as it links model-independent quantities. Coupled
with a strong anti-correlation between the V to hard X-ray flux ratio and the
H$\beta$ FHWM, it supports scenarios for the origin of the soft excess in AGN,
which require strong suppression of the hard X-ray emission; b) a strong (and
expected) correlation between the X-ray luminosity and the black hole mass. Its
slope, flatter than 1, is consistent with Eddington ratio-dependent bolometric
corrections, such as that recently proposed by Vasudevan & Fabian (2009).
Moreover, we critically review through various statistical tests the role that
distance biases play in the strong radio to X-ray luminosity correlation found
in CAIXA and elsewhere; we conclude that only complete, unbiased samples (such
as that recently published by Behar & Laor, 2008) should be used to draw
observational constraints on the origin of radio emission in radio-quiet AGN.
