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Soft X-ray Diffuse Background

Introduction

The nature of the soft X-ray diffuse background (SXRB) varies considerably over its energy range. At the lowest energies, 0.1 - 0.3 keV, nearly all of observed SXRB originates as thermal emission from hot (~106 K) plasma. There are two major components of this hot plasma. First, it is contained in a hot bubble in the disk of the Galaxy which surrounds the Sun (but was not created by the Sun) and extends from ~50 pc to ~200 pc in different directions. Second, there is an extensive distribution of this plasma in the halo of our Galaxy. Above 1 keV, most of the SXRB is not actually diffuse in origin but is rather the superposition of many unresolved discrete extragalactic sources, such as active galactic nuclei (AGN) and quasars. (We know this because with very long X-ray observations we can identify the individual sources.) Between 0.5 and 1 keV the situation is considerably more confused. Both extragalactic discrete sources and Galactic emission from hot plasma contribute to the observed flux. As extragalactic objects are discussed in other places, this text will concentrate on the low-energy Galactic diffuse emission.

Historical Background

Early X-ray Observations

The study of the 1/4 keV SXRB began in the late 1960s with sounding-rocket observations. From these first observations, the 1/4 keV background exhibited a surface brightness which was both intense and varying with direction. With the relatively crude angular resolution of these experiments, the most obvious feature was a general trend of greater intensity at high Galactic latitudes than in the plane of the Galaxy. Early investigators arrived at the logical conclusion that the SXRB originated outside of the Galaxy and that the variation of intensity was due to absorption by the neutral interstellar medium (ISM) of the Galactic disk (One optical depth at 1/4 keV is reached in ~100 parsecs in typical disk conditions.). The measured non-zero flux from the Galactic plane was attributed to an additional non-cosmic background component which could not be identified and removed.

However, with additional independent observations it became apparent that the flux observed in the Galactic plane was very likely to be cosmic (originating beyond the solar system) in origin. This and other inconsistencies of the "absorption model" (the postulation of an extragalactic source of emission absorbed by the Galactic ISM) were explained by having a local (nearest hundred parsecs), unabsorbed component. From this point, discussions of the origin of the SXRB became tightly linked to models of the local ISM.

Over the next two decades great strides were made in improving the quality, sky coverage, angular, and spectral resolution of the data. During this time, there was comparatively little disagreement about the "facts" of the data. Different groups presented data collected using different instruments acquired by different means, which were consistent with each other. By the mid-1990s there were four independent all-sky surveys in the 1/4 keV band, one from a campaign of sounding-rocket flights and three from satellite experiments. Figure 1 shows the 1/4 keV band map from the ROSAT survey. For comparison, Figure 2 shows a map of Galactic HI. The general negative correlation between the two data sets, dominated by the Galactic plane to high Galactic latitude variation, is readily apparent. Figure 3 shows the 3/4 keV band map from the ROSAT survey. Note how different the structure is from the 1/4 keV band map. The 3/4 keV data are relatively flat across the sky with the addition of distinct Galactic features. The largest is Loop I, the ~100 degree ring of enhanced emission in the Galactic center direction. This is thought to be a supernova remnant/stellar wind bubble at a distance of 150 parsecs and a radius of ~100 parsecs.

ROSAT All-Sky 0.25 keV band map
Neutral hydrogen in our galaxy
ROSAT All-Sky 0.75 keV band map
1/4 keV X-ray Background
(Figure 1)

Galactic Hydrogen Distribution
(Figure 2)

3/4 keV X-ray Background
(Figure 3)

While the X-ray data were in good agreement, the interpretation of those same data engendered often lively discussions. Models were proposed ranging from having most of the observed background originating within the nearest few hundred parsecs to having it originate over long path lengths even in the Galactic disk or in the Galactic halo and beyond.

Observations at Other Wavelengths

The late 1970s and 1980s saw considerable work in other energy ranges which had significant implications for our understanding of the local ISM. A local deficit in the neutral material of the Galactic disk was identified using 21-cm observations. ISM absorption line measurements of the spectra of relatively nearby stars were used to show conclusively that there is a local cavity in the HI of the Galactic disk which surrounds the Sun (but is unrelated to the Sun). The path lengths of low HI space density vary considerably even in the Galactic plane with values ranging from tens to hundreds of parsecs. Even the "cavity" was shown to be a complicated region with a partially ionized component of limited extent surrounding the Sun and significant path lengths of HII gas in at least one direction. Besides having regions of partially ionized and HII gas, the local cavity in the HI was a logical place to put the hot plasma responsible for the local component of the SXRB (that which is observed in the Galactic plane).

IRAS all-sky survey map
All-Sky survey map from IRAS

Data from IRAS (a satellite infrared observatory) have contributed considerably to our view of the ISM. While without the velocity information of 21-cm HI observations, the IRAS 100 micron data show extensive structure in the neutral material at much higher angular resolution than allowed by single-dish, 21-cm observations. The tight correlation between HI column density and IRAS 100um intensity at high Galactic latitudes demonstrated that the IRAS data could be used as a tracer of the total neutral and (with some limitations) molecular column density at a few arc minute resolution.

Current Model

By the end of the 1980s, the picture of the local ISM and its relationship to the SXRB was best described by the "displacement" model. This model postulates that the bulk of the observed 1/4 keV flux originates as diffuse emission from a thermal plasma at ~106 K which is contained within the local HI cavity. The negative correlation between HI column density and SXRB surface brightness is a natural result of the cavity being more extended out of the plane of the Galaxy, which includes more of the hot plasma and therefore produces more emission. While describing the relationship between NH and SXRB reasonably well, the model had the advantage of being reasonably consistent with the rest of the observational data. It placed the hot plasma in the HI void so there was no problem with too many components for the local ISM. "Bulk" is an important word here as there are other, obvious components to the SXRB such as SNRs which contributed emission over large solid angles (e.g., the Loop I Bubble) and non-obvious components such as some expected extragalactic emission from the low-energy extrapolation of the emission observed at higher energies. While we observe the local hot plasma so we know that it exists, the origin of the plasma is unknown. The most likely explanation is a supernova occurring over 100,000 years ago which reheated an existing cavity in the Galactic disk.

The major advance of the 1990s has been the conclusive discovery of hot (106 K) plasma in the halo of our Galaxy by the ROSAT project. While the local emission region still looks pretty much the same, we now know that up to half of the 1/4 keV emission observed at high Galactic latitudes originates beyond the neutral material of the Galactic disk. This halo emission varies considerably in different directions, but is nearly always present. Many questions still need to be answered about this component, for example: Where did it come from? How extensive is it? How long does it exist?

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