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The X-ray spectra of many active galactic nuclei (AGN) show a soft X-rayexcess below 1-2 keV on top of the extrapolated high- energy power law. Theorigin of this component is uncertain. It could be a signature ofrelativistically blurred, ionized reflection, or the high-energy tail ofthermal Comptonization in a warm (kT $\sim$ 1 keV), optically thick($\tau\simeq$ 10-20) corona producing the optical/UV to soft X-ray emission.The purpose of the present paper is to test the warm corona model on astatistically significant sample of unabsorbed, radio-quiet AGN with XMM-newtonarchival data, providing simultaneous optical/UV and X-ray coverage. The samplehas 22 objects and 100 observations. We use two thermal comptonizationcomponents to fit the broad-band spectra, one for the warm corona emission andone for the high-energy continuum. In the optical-UV, we also include thereddening, the small blue bump and the Galactic extinction. In the X-rays, weinclude a WA and a neutral reflection. The model gives a good fit (reduced$\chi^2 <1.5$) to more than 90% of the sample. We find the temperature of thewarm corona to be uniformly distributed in the 0.1-1 keV range, while theoptical depth is in the range $\sim$10-40. These values are consistent with awarm corona covering a large fraction of a quasi-passive accretion disc, i.e.that mostly reprocesses the warm corona emission. The disk intrinsic emissionrepresents no more than 20% of the disk total emission. According to thisinterpretation, most of the accretion power would be released in the upperlayers of the accretion flow.

Testing warm Comptonization models for the origin of the soft X-ray excess in AGN