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 disc 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 (star formation rate) ratio in the 0.5-2 keV band ranges from zero to ~1.8 × 10³⁸ (erg s^-1)/(M_solaryr^-1), 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 of 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.