The Contributions by ROSAT

After the launch of ROSAT, the picture changed significantly. The ROSAT PSPC All-Sky Survey (RASS) provided a new large-scale data base in the soft X-ray band with higher spatial and spectral resolution than available before, accompanied by an extensive follow-up programme of pointed observations.

The following major components of the SXRB have been identified since the early days, mainly with ROSAT data: (i) unresolved Galactic point sources (e.g., stars, accreting compact objects), (ii) unresolved extragalactic point sources, comprising about 80% of the total emission (e.g., AGN), (iii) diffuse local emission (Local Bubble, Loop I), (iv) diffuse distant Galactic emission (hot ISM and Galactic halo), (v) a diffuse warm-hot intergalactic medium (WHIM), and (vi) a yet unquantified very local emission component due to charge exchange reaction of solar wind ions with geocoronal and heliospheric gas.

Maps of the diffuse soft X-ray background (after point source removal) in several energy bands have been constructed (see Fig. 18.1). The morphology of the 1/4 and 3/4keV band maps reveals large differences: while the 3/4 keV (and also 1.5 keV) emission show generally a homogeneous emission except for the North Polar Spur (NPS) and the Sco-Cen region (in the general direction of the galactic center) the softer 1/4keV emission is full of small-scale and large-scale variations. It is much more intense at high latitudes, and still of the order of 1/3 in the galactic plane [63].

These maps can be combined to band ratio maps or to composite color images (see Fig. 18.2). Red color indicates soft emission while blue regions illustrate either hard emission or presence of strong absorption (e.g., in the galactic plane). Green indicates plasma emission related to supernova remnants and superbubbles.

The efficient background rejection of the ROSAT PSPC allowed for the first time a detailed imaging of the distribution of faint diffuse soft X-ray emission and allowed also the discovery of X-ray shadows. One of the first milestone exposures has been the observation of the crescent Moon, where the dark side casts a shadow onto distant X-ray emission, proving that the bulk of the soft X-rays were originating beyond the Moon [54]. Furthermore, in a pointed observation as well as in the ROSAT survey a shadow by the Draco cloud (distance —600 pc) has been detected in the 1/4 keV band [9, 59]. The detected X-ray flux showed an anticorrelation with the cloud content as traced by the 21 cm HI line and IRAS 100^m maps. For the first time clear evidence had been obtained that a major fraction (>50%) of the SXRB is emitted at large distances, and that the distribution of material responsible for the SXRB may be more complicated.

The ROSAT survey, however, suffered from the only moderate spectral resolution of proportional counters. A detailed spectroscopy was, therefore, not possible because of ambiguities in the spectral modelling. Various approaches were used to disentangle local and distant plasma components.

Looking toward opaque absorbers (e.g., dense molecular clouds) gives the foreground emission, off-cloud measurements sample both local and distant emission components: a comparison of both yields information on the distribution along the

Fig. 18.1 Top panel: ROSAT PSPC All-Sky Survey maps of the SXRB in three energy bands (top to bottom): 1/4, 3/4, 1.5keV. These maps have been used to construct the composite image in Fig. 18.2. Spatial resolution here is 20arcmin; point sources have been excluded. The galactic centre is at l = 0° with longitudes increasing to the left. The map production is described in [57], the details of the survey completion and these final maps can be obtained from [17]


Aitoff Projection Galactic tl Coordinate System

3-colour image: red: 0.1-0.4 keV green: 0.5-0.9 keV blue: 0.9-2.0 keV

Fig. 18.2 Multispectral X-ray view of the soft X-ray background as seen in the ROSAT PSPC All-Sky Survey. This RGB image covers three energy bands (red 1 /4keV, green 3/4 keV, and blue 1.5keV, respectively). The galactic centre is at l = 0° with longitudes increasing to the left ([17], and references therein)

line of sight. Another approach is by means of band ratio maps, the most sensitive one is the ratio of the two softest ROSAT bands.3 Variations of this band ratio away from known distinct emission features (e.g., supernova remnants, superbubbles) can be interpreted as spectral variations of foreground emission as optical depth of unity is already reached below NH < 1020 cm-2, at the Local Bubble boundary [64].

Further observations of high-latitude molecular clouds at known distances (e.g., MBM 12, [60]) and relations of other soft X-ray shadows indicated that the distant emission is distributed very inhomogeneously in the 1/4 keV band (cf. [63]). MBM 12 is one of the closest molecular clouds, at a distance similar or slightly larger than the extent of the Local Bubble.

One difficulty arises from the fact that the real zero level of the SXRB has to be known. "Noncosmic" contributions can be discriminated by means of temporal variations. In the RASS the PSPC (114arcmin field-of-view diameter) scanned the sky in great circles along constant ecliptic longitude with a survey progression of about 4 arcmin in the ecliptic plane per revolution. Therefore, each part of the sky has been observed multiple times in these largely overlapping scans. Time variations within a measured sky portion in spectrum and intensity could thus be discriminated and assigned to "noncosmic" origins. The most prominent ones were solar X-rays scattered off Earth's residual atmosphere along the line of sight, and "long-term

3 ROSAT bands R1 and R2, which together form the 1 /4 keV band, band ratio defined as R2/R1.

enhancements" lasting for several (4 - 9) h. The latter could be related to changes in solar wind parameters [15]. They are probably due to X-rays generated from charge exchange processes by the solar wind with neutral exospheric material (similar to X-rays produced at comets) [16]. Variations on time-scales of months or years (e.g., solar cycle) could not be determined by the only 6 months long RASS; these were proposed if heliospheric material were responsible. The measured zero-level of the SXRB may therefore only be an upper limit of the "true" zero-level. The future mission eROSITA [48] will scan the sky many times within 4 yrs and will, therefore, provide a much better database.

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