We present an analysis of the XMM- Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy and we discuss possible explanations for its very sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands .
The X-ray investigation shows a hard ( \Gamma = 1.89 ) and moderately absorbed ( N _ { H } = 1.8 \times 10 ^ { 21 } 
cm ^ { -2 } ) nuclear source of 1.5 \times 10 ^ { 40 } erg s ^ { -1 } in the 2–10 keV band , surrounded by hot gas at a temperature of \sim 0.6 
keV .
The bolometric nuclear luminosity is at least \sim 200 times lower than expected if mass accreted on the supermassive black hole , that HST shows to reside at the center of this galaxy , at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disc .
The low luminosity , coupled to the observed absence of Fe-K emission in the nuclear spectrum , indicates that such a disc is not present .
This nucleus differs from bright unobscured AGNs also for the lack of high flux variability and of prominent broad H \alpha emission .
However , it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate ; it could be too radio bright , instead , for radiatively inefficient accretion that includes strong mass outflows or convection .
This discrepancy could be solved by the possible presence of nuclear radio jets .
An alternative explanation of the low luminosity , in place of radiative inefficiency , could be unsteady accretion .