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Spectral Analysis

X-ray Spectroscopy

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 as well.

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.

Caveat

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.

* Use Hera to try your hand at analyzing spectra with modern data.


Thank you to Steve Holt, NASA-GSFC, for contributing to this article.

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This page last updated: Wednesday, 03-Feb-2010 15:10:38 EST