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

4. The 3/4 keV RASS data

In our analysis of the RASS data we subdivided the sky into galactic latitude strips of width. Fig. 3 shows, for example, the RASS keV and 1.5 keV data and the Leiden/Dwingeloo H I  data for two strips: to (upper and to lower). Before analyzing the intensity distribution of the keV count rate (Sects. 4.3 and 4.4), we first have to account for the contamination of the RASS data by point sources in the diffuse XRB maps (Sect. 4.1). Furthermore, we have to exclude individual galactic X-ray features such as Loop I (e.g. Egger & Aschenbach 1995) and the Orion-Eridanus Bubble (e.g. Brown et al. 1995 and Guo et al. 1995, Sect. 4.2).

Fig. 3. Two examples of the analyzed galactic latitude strips with width. In the upper panel the galactic latitude range between and is shown; the lower panel represents the latitudes between and . The upper row in each panel shows the raw data for the keV (left) and 1.5 keV RASS (middle) energy bands and the corresponding total H I  column densities (right). The scatter of the X-ray raw-data points stems from photon statistics and from the presence of X-ray point sources not subtracted from the XRB maps. In the lower row of each panel, the X-ray data is smoothed over galactic longitude, to a resolution of , after removing the brightest point sources. The smoothed data sets did not contain the X-ray contributions for Loop I or the Orion-Eridanus Bubble (grey-shaded area), which are visible as bright X-ray features in the raw keV and 1.5 keV maps.

4.1. Residual contamination of the RASS data

The keV map shown by Snowden et al. (1995) shows residual scanning effects, reaching X-ray intensities of about relative to the neighbouring areas. The comparison of the keV and 1.5 keV latitude strips shown in Fig.  3 reveals that the scatter of the data points is larger in the 1.5 keV band than in the keV one. Since the contribution of the extragalactic XRB resulting from the superposition of X-ray point sources is significantly larger at 1.5 keV than at keV (Sect. 3), we attribute this enhanced scatter to the presence of X-ray point sources which have not been subtracted. Since the count rates of the keV and 1.5 keV energy bands are almost the same, the difference in scatter is not caused by photon statistics.

Before converting our spectral fit results to the RASS data we have to consider the cumulative effect of the not-removed point sources on the survey data. To determine the level of the extragalactic XRB in the keV RASS data we analyzed 22 individual PSPC pointed observations, distributed in the range , to . Towards sky areas where these pointed PSPC data showed no significantly-detected point sources we evaluated the count rate of the XRB in the keV and 1.5 keV energy bands. Accounting for the lower angular resolution of the RASS data we evaluate a constant count-rate offset between the pointed PSPC data and the RASS maps. The RASS data reveal a count-rate offset of , generally higher than that of the pointed observations. This enhanced XRB level of the RASS data relative to the PSPC pointings, can most likely attributed to not-subtracted point sources. Due to the angular smoothing procedure applied to the RASS data we can assume that this count-rate offset is constant across the sky. Henceforward we use to represent the constant XRB residual point-source offset for the RASS data relative to the pointed PSPC observation analysis.

Now we can evaluate the keV XRB intensity level for the RASS. As shown in Sect. 3, the pointed PSPC data give . We add now the residual-point-source intensity level, , and arrive thus at for the RASS data set.

Before starting the analysis of the distant galactic XRB component in the keV RASS data, individual extended X-ray features need to be excluded from further analysis, since we are interested in the smooth, undisturbed soft XRB distribution. Fig. 3 shows that in the general direction of the inner Galaxy ( to ) the keV and 1.5 keV count rates are anomalously high. Most of this excess emission is associated with the Loop I shell (e.g. Egger & Aschenbach 1995). A second component may be attributable to emission from the galactic bulge itself. Another prominent X-ray region is located in the general direction of the galactic anticenter, at to , namely the Orion-Eridanus Bubble (e.g. Brown et al. 1995, Guo et al. 1995). In the following, Loop I and/or the galactic bulge region will be masked out completely, whereas the Orion/Eridanus Bubble will be marked by a grey area in the diagrams (see Fig. 3).

4.2. Dependence of the 3/4 keV amplitudes on galactic longitude

The variation of the keV and 1.5 keV count rates with galactic longitude (Fig. 3) suggests - if the individual features mentioned in the previous section are ignored - that the 1.5 keV count rate is constant, whereas the keV rate varies with longitude. The lowest keV count rates are reached at , suggesting a galactic origin for a significant fraction of the keV radiation.

The foreground X-ray gas with a temperature of kT 0.08 keV (Kerp 1994, Sidher et al. 1996) contributes only negligibly to the total keV count rate. This conclusion is supported by an anti-correlation analysis of the keV data of PSPC pointings toward , where we obtain a keV foreground component of . Moreover, since more than 85% of the total H I  column density is located below (e.g. Lockman & Gehman 1991), we conclude that a substantial portion of the keV radiation originates beyond the bulk of neutral hydrogen towards high galactic latitudes. Photoelectric absorption by the interstellar matter on the line of sight thus has to be taken into account.

Towards high latitudes the is typically about 1 - 2 ; therefore the attenuation of the keV XRB is about 12%. To account for the observed intensity contrast (see e.g. Fig. 5, ) between the galactic anticenter region and the directions towards the galactic center we need a column-density contrast of across large angular scales. Such a large contrast is not observed for the warm H I  layer (Dickey & Lockman 1990) which accounts dominantly for the photoelectric absorption (Kerp & Pietz 1996). Therefore the photoelectric absorption traced by H I  cannot account for the observed keV intensity variation.

Absorption across large angular scales by molecular hydrogen can be neglected, because at high latitudes () the surface filling factor of H₂ is only about 0.005 (e.g. Magnani et al. 1996). The layer (Reynolds 1991) can also not account for the keV intensity variation because its maximum column density is about and consequently absorbs the X-ray radiation insignificantly in comparison to the H I .

We conclude that the observed variation of keV intensities with galactic longitude is not caused by photoelectric absorption. Therefore, the isotropic intensity distribution of the extragalactic XRB is not the source of the observed intensity variation with galactic longitude. Most probably the observed keV intensity variation is caused by the intensity variation of the distant galactic XRB source. Towards the galactic center the distant galactic XRB source is bright while towards the outskirts of the galactic disk the intensity decreases.

In view of the above we can simplify the radiation transport equation for the keV energy range to:

where and denote the effective photoelectric absorption cross sections.

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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