An extended galactic population of low-luminosity x-ray sources (CVs?) and the diffuse x-ray background

Summary of abstract: The existence of a new population of yet unrecognizedx-ray sources has been often suggested to resolve some pecularities in theproperties of the x-ray background (XRB), and has recently been indicated by ananalysis of resolved sources in deep ROSAT observations (Hasinger \etal 1993). We investigate the possibility that the indicated new population of x-raysources is Galactic in origin, and derive its properties which would resolvethe discrepancy found in the number counts of faint sources, and be consistentwith observational constraints on the total background intensity, the XRBanisotropy, the number of unidentified bright sources, the Galaxy's total x-rayluminosity, and with the results of fluctuation analyses of the unresolved XRB. We find that a flattened Galactic halo (or a thick disk) distribution with ascale height of a few Kpc satisfies all the above requirements. The typicalx-ray luminosity of the sources is $\approx 10^{30-31}$ erg/s in the 0.5-2 KeVband, the number density of sources in the Solar vicinity is $\sim 10^{-4.5}pc^{-3}$, their total number in the Galaxy is $\sim 10^{8.5}$, and theycontribute $\sim 10^{39}$ erg/s to the Galaxy's total x-ray luminosity. We discuss the possible nature of these sources (subdwarfs, LMXBs, oldneutron stars), and argue that the inferred x-ray and optical luminosities ofthe sources, their $\sim 2-4$ KeV spectrum, and the derived local numberdensity and spatial distribution are all consistent with them being intrinsiclyfaint cataclysmic variables with low accretion rates. We discuss the possibleorigin of such population, including an origin from disrupted globular clustersor dark clusters, make predictions, and suggest observational tests.Comment: 20 pages, PostScript file (330K including figures), submitted to Ap