2004A&A...427...35P

We report the discovery of a Compton-thick AGN and of intense star-formation activity in the nucleus and disk, respectively, of the nearly edge-on superwind galaxy NGC 4666. Spatially unresolved emission is detected by BeppoSAX only at energies <10keV, whereas spatially resolved emission from the whole disk is detected by XMM-Newton. A prominent (EW∼1-2keV) emission line at ∼6.4keV is detected by both instruments. From the XMM-Newton data alone the line is spectrally localized at E≃6.42 ±0.03keV, and seems to be spatially concentrated in the nuclear region of NGC 4666. This, together with the presence of a flat (Γ∼1.3) continuum in the nuclear region, suggests the existence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic 2-10keV luminosity is estimated to be L_2–10>2x10⁴¹erg/s. At energies ≲1keV the integrated (BeppoSAX) spectrum is dominated by a ∼0.25keV thermal gas component distributed throughout the disk (resolved by XMM-Newton). At energies ∼2-10keV, the integrated spectrum is dominated by a steep (Γ>2) power-law (PL) component. The latter emission is likely due to unresolved sources with luminosity L∼10³⁸-10³⁹erg/s that are most likely accreting binaries (with BH masses ≤8M_☉). Such binaries, which are known to dominate the X-ray point-source luminosity in nearby star-forming galaxies, have Γ∼2PL spectra in the relevant energy range. A Γ∼1.8PL contribution from Compton scattering of (the radio-emitting) relativistic electrons by the ambient FIR photons may add a truly diffuse component to the 2-10keV emission.