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We report the discovery of a Compton-thick AGN and of intense star-formationactivity in the nucleus and disk, respectively, of the nearly edge-on superwindgalaxy NGC 4666. Spatially unresolved emission is detected by BeppoSAX only atenergies <10 keV, whereas spatially resolved emission from the whole disk isdetected by XMM-Newton. A prominent (EW ~ 1-2 keV) emission line at ~6.4 keV isdetected by both instruments. From the XMM-Newton data alone the line isspectrally localized at E ~ 6.42 +/- 0.03 keV, and seems to be spatiallyconcentrated in the nuclear region of NGC 4666. This, together with thepresence of a flat (Gamma ~ 1.3) continuum in the nuclear region, suggests theexistence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic2-10 keV luminosity is estimated to be L_{2-10} > 2 x 10^{41} erg/s. Atenergies <1 keV the integrated (BeppoSAX) spectrum is dominated by a ~0.25 keVthermal gas component distributed throughout the disk (resolved by XMM-Newton).At energies ~2-10 keV, the integrated spectrum is dominated by a steep (G > 2)power-law (PL) component. The latter emission is likely due to unresolvedsources with luminosity L ~ 10^{38} - 10^{39} erg/s that are most likelyaccreting binaries (with BH masses <8 M_sun). Such binaries, which are known todominate the X-ray point-source luminosity in nearby star-forming galaxies,have Gamma ~ 2 PL spectra in the relevant energy range. A Gamma ~ 1.8 PLcontribution from Compton scattering of (the radio-emitting) relativisticelectrons by the ambient FIR photons may add a truly diffuse component to the2-10 keV emission.

The composite starburst/AGN nature of the superwind galaxy NGC 4666