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.
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).
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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