Unresolved X-ray emission in M31 and constraints on progenitors of
  Classical Novae
  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 L_X/SFR ratio in the 0.5-2 keV band ranges
from zero to ~1.8 x 10^38 (erg/s)/(M_sun/yr), excluding the regions near the
minor axis of the galaxy where it is ~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 ~10 per cent of CNe can be produced in magnetic cataclysmic variables, the
upper limit being ~3 per cent for parameters typical for CN progenitors. In
dwarf novae, >~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.
