X-ray spectroscopy offers an important channel of information about our
Universe. It has achieved increasing importance as X-ray astronomy has
matured and technology has developed. Now, we have the ability to make
measurements not just of the X-ray continua, but of discrete line features
X-ray Spectral Categorization
The wide variety of X-ray sources present a wide variety of
observations. In some cases, X-ray spectra provide unique opportunities to gain
insight into the nature of the source.
Since line emission ceases to be the dominant cooling mechanism of
astrophysical plasmas for temperatures exceeding 1 keV, X-ray spectra are
primarily characterized by their continua. The simplest of these are the
featureless power laws produced by the interaction of power law
distributions of cosmic ray electrons with ambient magnetic fields. The
Crab nebula, for example, is a prime example of such a source, called a
synchrotron source. Almost as simple is the blackbody spectrum, which
characterizes the other extreme, i.e. it is the result of interactions
between particles and photons such that complete thermalization has occurred.
Such spectra can be seen, for example, in X-ray bursts generated by nuclear
burning episodes on the surfaces of neutron stars.
Most X-ray sources usually exhibit "intermediate" continua in the
sense that electron scattering plays a role in the formation of the observed
spectra. This is true for the modified bremsstrahlung spectra of galactic
binary systems which contain white dwarfs, neutron stars, or black holes, as
well as for active galaxies. Emission line features have also been seen in
both types of objects.
In fact, the optically thin thermal spectra at X-ray temperatures of a
variety of astrophysical system types provide rich line spectra for
X-ray spectroscopists. In many systems, the X-ray emitting plasma is
sufficiently transparent that the emergent spectrum faithfully represents the
microscopic processes occurring in the plasma. Such spectra offer the
possibility of deducing many properties of the emitting gas (given
sufficient detector resolving power and sensitivity). For example, the
elemental composition and abundances, the temperature, and electron
temperature and density can all be gleaned from analysis of the line spectra.
The problem of translation from photons detected by a spectrometer
to a cosmic source spectrum is not trivial, and it is important to
recognize that any interpretation of an observation can depend on the
analysis procedure employed by the astronomer. The raw data are a
convolution of the actual input photon spectrum with the response
function of the spectrometer; deconvolution to determine the input photon
spectrum is not necessarily unique.
The conventional method is a model-dependent procedure which requires that
the astronomer have some a priori knowledge of the actual spectral form so
that it can be characterized by a limited number of adjustable parameters.
Typically, the fitting parameters include amplitude and shape of the
continuum, strengths (and possibly energies) of emission lines and
photoelectric absorption edges, and low energy photoabsorption by
intervening cold matter. Simulated detector count spectra are computed from
assumed spectral forms, and the model parameters are varied to achieve the
best fit to the actual detector counts. There are good things and bad things
about such an approach: The Good - spectral features blurred by the
detector response can be enhanced for display; The Bad - unanticipated
features are forced to be represented by the assumed parameters.
Thank you to Steve Holt, NASA-GSFC, for contributing to this article.