Abstract
view Abstract Citations (10) References (65) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Extended Galactic Population of Low-Luminosity X-Ray Sources (CVs?) and the Diffuse X-Ray Background Maoz, Eyal ; Grindlay, Jonathan E. Abstract Summary of abstract: The existence of a new population of yet unrecognized x-ray sources has been often suggested to resolve some pecularities in the properties of the x-ray background (XRB), and has recently been indicated by an analysis of resolved sources in deep ROSAT observations (Hasinger \etal 1993). We investigate the possibility that the indicated new population of x-ray sources is Galactic in origin, and derive its properties which would resolve the discrepancy found in the number counts of faint sources, and be consistent with observational constraints on the total background intensity, the XRB anisotropy, the number of unidentified bright sources, the Galaxy's total x-ray luminosity, and with the results of fluctuation analyses of the unresolved XRB. We find that a flattened Galactic halo (or a thick disk) distribution with a scale height of a few Kpc satisfies all the above requirements. The typical x-ray luminosity of the sources is $\approx 10^{30-31}$ erg/s in the 0.5-2 KeV band, 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 they contribute $\sim 10^{39}$ erg/s to the Galaxy's total x-ray luminosity. We discuss the possible nature of these sources (subdwarfs, LMXBs, old neutron stars), and argue that the inferred x-ray and optical luminosities of the sources, their $\sim 2-4$ KeV spectrum, and the derived local number density and spatial distribution are all consistent with them being intrinsicly faint cataclysmic variables with low accretion rates. We discuss the possible origin of such population, including an origin from disrupted globular clusters or dark clusters, make predictions, and suggest observational tests. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175593 arXiv: arXiv:astro-ph/9408100 Bibcode: 1995ApJ...444..183M Keywords: Background Radiation; Diffuse Radiation; Galactic Structure; Milky Way Galaxy; Populations; X Ray Astronomy; X Ray Sources; Cataclysmic Variables; Faint Objects; Galactic Evolution; Galactic Halos; Neutron Stars; X Ray Stars; Astronomy; COSMOLOGY: DIFFUSE RADIATION; GALAXIES: STELLAR CONTENT; GALAXY: HALO; STARS: NOVAE; CATACLYSMIC VARIABLES; X-RAYS: GALAXIES; X-RAYS: STARS; Astrophysics E-Print: 20 pages, PostScript file (330K including figures), submitted to ApJ full text sources arXiv | ADS |
Highlights
More than 30 years after the discovery of the x-ray background (Giacconi et al 1962) the origin of this radiation is still not entirely understood
Examining spherical halo and thick disk distributions, we derive the allowed properties of such population which would resolve the discrepancy found in the number counts of faint sources, and be consistent with observational constraints on the total background intensity, the x-ray background (XRB) anisotropy, the number of unidenti ed bright sources, the Galaxy's total x-ray luminosity, and with the results of uctuation analyses of the unresolved XRB
Mather et al (1990) have shown that thermal Bremsstrahlyung from a di use hot medium cannot provide a signi cant fraction of the XRB, leaving the alternative possibility that the XRB arises predominantly from an unresolved population of discrete sources. This is supported by uctuation analyses which show that the contribution of discrete sources to the '1-2KeV background is > 75-90%, and it is consistent with the entire XRB being due to discrete sources (e.g., Hamilton & Helfand 1987; Soltan 1991; Shanks et al 1991; Hasinger et al 1993)
Summary
More than 30 years after the discovery of the x-ray background (Giacconi et al 1962) the origin of this radiation is still not entirely understood. Mather et al (1990) have shown that thermal Bremsstrahlyung from a di use hot medium cannot provide a signi cant fraction of the XRB, leaving the alternative possibility that the XRB arises predominantly from an unresolved population of discrete sources This is supported by uctuation analyses which show that the contribution of discrete sources to the '1-2KeV background is > 75-90%, and it is consistent with the entire XRB being due to discrete sources (e.g., Hamilton & Helfand 1987; Soltan 1991; Shanks et al 1991; Hasinger et al 1993). Fluctuation analyses (Barcons & Fabian 1988; Persic et al 1989; Carrera & Barcons 1992; Carrera et al 1993; Soltan & Hasinger 1994) severely constrain the clustering properties of sources producing the XRB, regardless of their nature They have shown that sources which are clustered like galaxies or like the optically observed QSOs cannot provide more than about 60-65% of the total background intensity. In x5 we summarize the results, make predictions, and propose tests
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