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

3. Spectral components of the XRB

In this section we decompose the XRB radiation into its individual spectral components. As mentioned in the introduction, the excess soft X-ray emission below 1 keV can be explained by absorbed background radiation emitted by thermal plasma in the temperature range of kT = 0.14-0.16 keV (Gendreau et al. 1995). Rocchia et al. (1984) found evidence for line emission in XRB spectra, supporting the assumption that a significant part of the XRB is of thermal origin.

To investigate the spectral composition of this emission we need to use at least three source terms when fitting the PSPC X-ray spectra: the first of these terms represents the contribution from a local foreground gas, denoted as ; the second term, , represents the extragalactic XRB radiation; and the third term, , represents the distant galactic X-ray plasma in our Galaxy.

In order to evaluate the physical parameters of the different XRB components we selected several high-galactic-latitude PSPC pointings near (Table 1). The selected PSPC pointings cover an H I  column-density range of . In accordance with the accumulating evidence that the distribution of the soft X-ray sky is anti-correlated to the H I  column density distribution (see Kerp et al. 1997), a high H I  column-density contrast yields a high X-ray intensity contrast. The selected pointings avoid areas close to the well-known radio-continuum loops (e.g. Berkhuijsen 1971).

Table 1. Central positions as well as the total and analyzed exposure times of the PSPC pointed observations near . These PSPC observations were analyzed to disclose the spectral composition of the XRB. The column gives the available total integration time; denotes the analyzed time after correction for the non-cosmic backgrounds.

Fits to ROSAT X-ray spectra allow separating the thermal and the extragalactic XRB components superposed in the broad energy bands of the keV and keV RASS data. Furthermore, this spectral analysis constrains the temperature of the distant galactic X-ray plasma.

Fig. 1a-c. X-ray spectra derived from the PSPC pointed observations (Table 1). The X-ray absorbing column density varies from at (left), to = at (center), and then up to at (right). The individual X-ray spectra were approximated by a model composed of three X-ray source components. A foreground X-ray component represents the local hot interstellar medium with a plasma temperature of about k keV (dotted line); a distant plasma component has a temperature of about k keV (dashed line); and the extragalactic X-ray background radiation is represented by a power-law X-ray spectrum with (Gendreau et al. 1995, dot-dashed line). All spectra were fitted with the same parameterization of the X-ray source components: we found a consistent solution for the XRB spectrum (solid line) over this large range of absorbing column densities observed towards high galactic latitudes.

In order to determine the distribution of absorbing column densities in detail we observed the H I  distribution with the Effelsberg telescope, selecting several areas within the individual ROSAT pointings optimized to cover a large range of H I  column densities. Since no ¹² CO () emission is detected towards the cloud in question (Heiles et al. 1988, Reach et al. 1994), and since the observed H I  column-density distribution reproduces the observed IRAS 100-µm intensity variations, at least down to a angular resolution, molecular gas as an additional X-ray absorber can be neglected.

We modelled the radiation transfer of X-ray absorbing and emitting gas according to the scheme: extragalactic and distant galactic XRB emission absorber local X-ray plasma observer For the spectral fitting procedure we used the Mewe-Kaastra (MK) X-ray plasma code (Mewe et al. 1985) and the photoelectric absorption cross sections of Morrison & McCammon (1983). We fit three source components to the X-ray spectra: a foreground MK plasma, an absorbed distant MK plasma, and an absorbed extragalactic photon power law (PL). To minimize the number of free parameters, we fixed the foreground plasma temperature to k keV and provided an ubiquitous absorbing column density of (Kerp 1994). The temperature of the local X-ray plasma is derived from the X-ray band ratio of the R1 (PI channels 11-19) and R2 energy band (PI channels 20-41), obtained towards high column density () areas, where the ISM is optically thick for the background 1/4 keV emission. Our temperature value is consistent with other publications (e.g. Sidher et al. 1996). An extragalactic power-law photon index of was used; this choice of the spectral index follows Gendreau et al. (1995) but is also consistent with the results of Almaini et al. (1996), who found that the photon index decreases towards fainter X-ray point source fluxes: at . Because our analysis deals with ROSAT PSPC data which have an average X-ray flux level fainter than , we adopt the power-law index as given by Gendreau et al. (1995). We optimized the spectral fitting procedure to fit simultaneously all three X-ray spectra with the same emission measures as well as with the same plasma temperatures. We stress that the differences between the X-ray spectra shown in Fig. 1 follow solely from the very different H I  column densities attenuating the distant X-ray components.

As the best-fit temperature for the distant X-ray plasma we obtained kT = 0.135 keV, a value in agreement with the results of others (e.g. Kerp 1994, Sidher et al. 1996) and close to the low X-ray plasma temperature range found by Gendreau et al.(1995).

The derived X-ray plasma parameters were additionally checked using an anti-correlation analysis comparing the keV radiation and the H I  column density. We plotted the 1/4 keV count rate versus in Fig. 2 and fitted this scatter diagram by a model based on two X-ray source components: a foreground count rate and an absorbed background count rate .

This approach is justified because the absorption factors of the thermal plasma and the power-law spectra, , are equal within the statistical uncertainties of the X-ray data. Therefore, we evaluate the sum of the extragalactic background component, , and the thermal galactic component, . This sum of both count rates, , corresponds to the emission measure of the distant plasma and gives the amplitude of the power-law component in the spectral fits. Therefore, the radiation transport equation for the keV energy range can be simplified to:

The resulting intensities are (see Fig. 2):

These values are consistent with the mean intensities derived from spectral fits converted into count rates of the keV band: for the foreground component, and for the sum of both distant XRB components. This quantitative agreement between the spectral fits and the scatter-diagram analysis indicates that the derived emission measures and plasma temperatures are well determined.

Fig. 2. Anti-correlation diagram of keV count rate versus the total H I  column density for the pointed PSPC observations (Table 1). The line indicates the best-fit solution for a two-source component model with a foreground count rate of and a background rate of . Within the ROSAT keV band the transmission of the distant plasma and of the extragalactic background photons differ by only about 7%; we can therefore approximate the superposition of both X-ray source terms by a single component. Thus, the two-component model fits the observed situation well.

In the above, we have neglected the influence of the ionized interstellar medium on the photoelectric absorption of the X-ray radiation. If we assume that the ionized galactic hydrogen is located in front of the distant galactic XRB component, a mean absorbing column density of has to be added as additional absorber towards the field of interest (see Reynolds 1991). Using the mean photoelectric absorption cross section of the ionized gas layer given by Snowden et al. (1994b) and the anti-correlation method described above, we obtain values of and .

The keV count rates are much less influenced than the keV count rates by ionized gas with column densities . Therefore, the ratio between the 3/4 keV background count rate and the corresponding keV count rate would increase if were added to the absorbing column density. Since the power-law contribution is determined by the high-energy part of the spectrum, this yields a slightly cooler X-ray background plasma temperature as evaluated by the spectral fitting procedure above. Based on this consideration we estimate that the distant galactic X-ray plasma temperature is kT = 0.13 - 0.14 keV.

Using this temperature we can now extrapolate the intensity values of the keV scatter analysis to the keV energy band and can disentangle the contributions of and of to . Approximately 35% of the total unabsorbed keV count rate, , is caused by the background plasma component. Consequently, the contribution of the extragalactic XRB component is about , consistent with the spectral fitting results of . In the 1.5 keV energy range, less than of the total observed radiation can be attributed to the distant galactic plasma component.

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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