We investigate unresolved X-ray emission from M31 based on an extensive set of archival XMM-Newton and Chandra data .
We show that extended emission , found previously in the bulge and thought to be associated with a large number of faint compact sources , extends to the disk of the galaxy with similar X-ray to K-band luminosity ratio .
We also detect excess X-ray emission associated with the 10-kpc star-forming ring .
The \mathrm { L _ { X } / SFR } ratio in the 0.5 - 2 keV band ranges from zero to \approx 1.8 \cdot 10 ^ { 38 } \mathrm { ( erg s ^ { -1 } ) / ( M _ { \odot } / yr ) } , excluding the regions near the minor axis of the galaxy where it is \sim 1.5 - 2 times higher .
The latter is likely associated with warm ionized gas of the galactic wind rather than with the star-forming ring itself .

Based on this data , we constrain the nature of Classical Nova ( CN ) progenitors .
We use the fact that hydrogen-rich material , required to trigger the explosion , accumulates on the white dwarf surface via accretion .
Depending on the type of the system , the energy of accretion may be radiated at X-ray energies , thus contributing to the unresolved X-ray emission .
Based on the CN rate in the bulge of M31 and its X-ray surface brightness , we show that no more than \sim 10 per cent of CNe can be produced in magnetic cataclysmic variables , the upper limit being \sim 3 per cent for parameters typical for CN progenitors .
In dwarf novae , \gtrsim 90 - 95 per cent of the material must be accreted during outbursts , when the emission spectrum is soft , and only a small fraction in quiescent periods , characterized by rather hard spectra .