NASA Goddard Space Flight Center
Welcome to Soft X-ray Diffuse Background Home Page at the Laboratory for High
Energy Astrophysics
(LHEA),
at NASA's
Goddard Space Flight Center
(GSFC)
The cosmic soft X-ray diffuse background (SXRB, E~0.1-1.5 keV) is many things to many people. To most it is a source of contamination which must be removed before their sources can be analyzed properly. To the true cognoscente among us, the SXRB is a source of unique information on the local interstellar medium, the Galactic halo, general Galactic structure, and cosmological evolution.
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, one component is contained within 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 (this region is known as the Local Hot Bubble). 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. While there is apparently relatively little local emission at 3/4 keV from the Local Hot Bubble, the Milky Way has extensive 3/4 keV emission regions throughout the disk of the Galaxy and apparently into the halo.
As extragalactic objects are discussed in other places, this text will concentrate on diffuse Galactic emission, mostly at 1/4 keV.
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.
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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.
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.
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. A recent discovery by Knie et al. (1999, Physical Review Letters, 85, 18) may provide us with the long-sought ``smoking gun.'' They were able to find 60Fe in a sample of deep-ocean ferromagnetic crust. This radioactive element with a half life of ~1.5 million years has no significant production paths other than in supernovae, and so can be used as a supernova ``tracer.'' The crusts examined to find the 60Fe both grow relatively slowly and can have their growth rates determined by measurement. Thus, finding an excess of 60Fe at a certain level in the crust allows the determination of how recently the material was deposited. In this case, the data indicate that a supernova exploded about five million years ago at a distance of 30 parsecs. Such a supernova is completely capable of leaving the hot plasma that we observe today.
The second part of the current model concerns X-ray emission in the halo of the Milky Way. To be more precise since different people have different views of what the Galactic halo is, the ``halo'' referred to here is anywhere above the bulk of the neutral material of the Galactic disk. The early ROSAT results showed very conclusively that there is extensive emission at 1/4 keV above the Galactic disk, occasionally intrinsically much brighter that the emission which surrounds the Sun. However, further work has shown that this emission is quite patchy over the sky, which also suggests that the emission originates relatively close to the 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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