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

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 <10 keV, whereas spatially resolved emission from the whole disk is detected by XMM-Newton. A prominent (EW ∼ 1-2 keV) emission line at ∼6.4 keV is detected by both instruments. From the XMM-Newton data alone the line is spectrally localized at E ≃ 6.42 ± 0.03 keV, 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 2-10 keV luminosity is estimated to be L_2-10 ≳ 2 × 10⁴¹ erg s^-1. At energies ≲1 keV the integrated (BeppoSAX) spectrum is dominated by a ∼0.25 keV thermal gas component distributed throughout the disk (resolved by XMM-Newton). At energies ∼2-10 keV, 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 ∼ 10³⁸-10 ³⁹ erg s^-1 that are most likely accreting binaries (with BH masses ≤8 M₃). Such binaries, which are known to dominate the X-ray point-source luminosity in nearby star-forming galaxies, have Γ ∼ 2 PL spectra in the relevant energy range. A Γ ∼ 1.8 PL contribution from Compton scattering of (the radio-emitting) relativistic electrons by the ambient FIR photons may add a truly diffuse component to the 2-10 keV emission.