Soft band X/K luminosity ratios for gas-poor early-type galaxies

Aims: We aim to place upper limits on the combined X-ray emission from the population of steady nuclear-burning white dwarfs in galaxies. In the framework of the single-degenerate scenario, these systems, known as supersoft sources, are believed to be likely progenitors of Type Ia supernovae.
Methods: From the Chandra archive, we selected normal early-type galaxies with the point source detection sensitivity better than 10³⁷ erg s^-1 in order to minimize the contribution of unresolved low-mass X-ray binaries. The galaxies, contaminated by emission from ionized ISM, were identified based on the analysis of radial surface brightness profiles and energy spectra. The sample was complemented by the bulge of M 31 and the data for the solar neighborhood. To cover a broad range of ages, we also included NGC 3377 and NGC 3585 which represent the young end of the age distribution for elliptical galaxies. Our final sample includes eight gas-poor galaxies for which we determine L_X/L_K ratios in the 0.3-0.7 keV energy band. This choice of the energy band was optimized to detect soft emission from thermonuclear-burning on the surface of an accreting white dwarf. In computing the L_X we included both unresolved emission and soft resolved sources with the color temperature of kT_bb ≤ 200 eV.
Results: We find that the X/K luminosity ratios are in a rather narrow range of (1.7-3.2) × 10²⁷ erg s^-1 L_K,⊙. The data show no obvious trends with mass, age, or metallicity of the host galaxy, although a weak anti-correlation with the Galactic NH appears to exist. It is much flatter than predicted for a blackbody emission spectrum with temperature of ~ 50-75 eV, suggesting that sources with such soft spectra contribute significantly less than a half to the observed X/K ratios. However, the correlation of the X/K ratios with NH has a significant scatter and in the strict statistical sense cannot be adequately described by a superposition of a power law and a blackbody components with reasonable parameters, thus precluding quantitative constraints on the contribution from soft sources.