AN EXTENDED GALACTIC POPULATION OF LOW-LUMINOSITY X-RAY SOURCES (CVS)AND THE DIFFUSE-X-RAY BACKGROUND

The incompatibility of the properties of the X-ray background (XRB) wi th AGNs contributing greater than or similar to 60% at energies of a f ew keV has often been interpreted as being due to a substantial contri bution of a new population of yet unrecognized X-ray sources. The exis tence of such population has been recently suggested also by an analys is of very deep ROSAT observations which revealed a considerable exces s of faint X-ray sources over that expected from QSO evolution models, and that the average spectrum of the resolved sources becomes harder with decreasing flux limit. These sources could be extragalactic in or igin, but if they make a substantial contribution to the XRB then they must exhibit much weaker clustering than galaxies or QSOs in order to be consistent with the stringent constraints on source clustering imp osed by autocorrelation analyses of the unresolved XRB. We investigate the possibility that the indicated new population of X-ray sources is Galactic in origin. Examining spherical halo and thick disk distribut ions, we derive the allowed properties of such populations which would resolve the discrepancy found in the number counts of faint sources a nd be consistent with observational constraints on the total backgroun d intensity, the XRB anisotropy, the number of unidentified bright sou rces, the Galaxy's total X-ray luminosity, and with the results of flu ctuation analyses of the unresolved XRB. We find that a flattened Gala ctic halo (or a thick disk) distribution with a scale height of a few kpc is consistent with all the above requirements. The typical X-ray l uminosity of the sources is approximate to 10(30-31) ergs s(-1) in the 0.5-2 keV band, the number density of sources in the solar vicinity i s similar to 10(-4.5) pc(-3), their total number in the Galaxy is simi lar to 10(8.5), and their total contribution to the Galaxy's X-ray lum inosity is similar to 10(39) ergs s(-1). We discuss the possible natur e of these sources, including their being subdwarfs, LMXBs, massive bl ack holes, and old neutron stars. We argue that the inferred X-ray and optical luminosities of the sources, the slope of their energy spectr um, and the derived local number density and spatial distribution are all consistent with their being intrinsically faint cataclysmic variab les with low accretion rates. We suggest a few possibilities for the o rigin of such population, including an origin from disrupted globular clusters or dark clusters. We make predictions and suggest tests that could either confirm or rule out our proposal in the near future.