Astron. Astrophys. 332, 55-70 (1998)

1. Introduction

The X-ray background (XRB) can be conveniently subdivided into several energy ranges where the spectral properties and consequently also the origin of the X-ray radiation are different. At energies above 1 keV, the XRB radiation is dominantly of extragalactic origin (Fabian & Barcons 1992, Hasinger et al. 1993) and reveals, accordingly, an isotropic intensity distribution across the sky. Gendreau et al. (1995) used ASCA data to show that the extragalactic XRB spectrum between 1 and 7 keV can be approximated by a power-law with a photon index, , of about -1.4. Moreover, they found evidence for excess X-ray emission, deviating from this power-law, at energies below 1 keV. Gendreau et al. (1995) fit this excess soft X-ray radiation using a thermal plasma with a temperature in the range kT = 0.14-0.16 keV. Soltan et al. (1996) also found evidence for a change in the spectral properties of the XRB below 1 keV in an autocorrelation analysis of the ROSAT all-sky survey. Nousek et al. (1982) had earlier come to similar conclusions by analyzing X-ray data obtained with non-imaging X-ray detectors; they favored (in particular for the keV range) an XRB model consisting of two components, one extragalactic and one with its origin in the halo of our Galaxy. The X-ray radiation of both contributions is attenuated by photoelectric absorption caused by the intervening interstellar matter distributed along the line of sight. However, the X-ray data analyzed by Nousek et al. (1982) did not allow them to distinguish in detail between different models of the XRB.

Modeling the XRB is more complicated in the keV energy range than at higher photon energies. Roughly half of the observed keV X-ray radiation can be attributed to thermal-plasma emission originating within the low-volume density environment surrounding the Sun, sometimes called the Local Hot Bubble (LHB; for a review see McCammon & Sanders 1990). The remaining half of the soft X-ray emission results from the superposition of the extragalactic XRB upon the diffuse galactic X-ray radiation arising at large distances in our own Galaxy (see Kerp et al. 1997).

In this paper we investigate the distant diffuse galactic soft X-ray radiation by correlating two new data sets: the Leiden/Dwingeloo H I  21-cm line survey (Hartmann & Burton 1997) and the public ROSAT all-sky survey (RASS, Snowden et al. 1995) covering the keV and keV energy ranges. Our aim is to improve the earlier analysis of Nousek et al. (1982) by analyzing more modern X-ray data obtained with an imaging X-ray detector, and by correlating these data with the new H I  data, in the realization that the distribution of the neutral interstellar gas largely determines the appearance of the soft X-ray sky. The data sets are briefly described in Sect. 2.

The RASS data are supplemented here by selected pointed PSPC observations, in order to address the contribution of the point sources to the XRB maps. Moreover, the PSPC pointings are used to derive the spectral composition of the XRB (Sect. 3). The spectral properties so determined are then used to solve the soft X-ray radiation transport equation for the keV and for the keV RASS data (Sect. 4). A modified isothermal flattened-halo X-ray model is presented by incorporating a radial scale length (Sect. 5). We compare this new galactic X-ray halo model with the keV RASS data in Sect. 6. We summarize our results in Sect. 7.

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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