Thermal X-ray radiation (PDF) - SRON

3.1 Introduction
Thermal X-ray radiation is an important diagnostic tool for studying cosmic sources
where high-energy processes are important. One can think of the hot corona of the
Sun and of stars, solar and stellar flares, supernova remnants, cataclysmic variables,
accretion disks in binary stars and around black holes (galactic and extragalactic),
the diffuse interstellar medium of our Galaxy or external galaxies, the outer parts of
active galactic nuclei (AGN), the hot intracluster medium, the diffuse intercluster
medium. In all these cases there is thermal X-ray emission or absorption.
We will see in this chapter that it is possible to derive many different physical
parameters from an X-ray spectrum: temperature, density, chemical abundances,
plasma age, degree of ionisation, irradiating continuum, geometry etc. In this chapter
we focus on X-ray emission and absorption in optically thin plasma’s. For optically
thick plasmas one needs to take account of the full radiation transport in
order to understand these plasmas. In that case stellar atmosphere models become
relevant. However, we will not treat those cases here and restrict our discussion to
plasmas with τ 1.
The power of high-resolution X-ray spectroscopy is shown in Fig. 3.1. This shows
a simulated spectrum of Capella based on old low-resolution observations with the
EXOSAT satellite. That the predictions work is shown in Fig. 3.2. This shows the
success of the plasma emission codes developed at SRON by Rolf Mewe (1935–2004)
and his colleagues.
The strength of spectral analysis lies often in the details. While there are many
spectral lines, some lines contain more information than others. Fig. 3.3 shows an
example of this. It is a part of the spectrum of a flare star. The three lines shown –
the famous Ovii triplet that we will encounter more often during this course – have
an enormous diagnostic power.
This triplet occurs in many other sources, with often totally different intensity
ratio. See for example Fig. 3.4 and compare the intensities of the triplet with those
in Fig. 3.3. While EQ Peg is in collisional ionisation equilibrium, NGC 1068 is in
photoionisation equilibrium. These terms will be explained later.
Up to now you only saw examples of emission spectra. A nice example of an
absorption spectrum is shown in Fig. 3.5.
In order to see thermal imprints on a spectrum sometimes requires the skills
of a spectroscopist. See for example Fig. 3.6. There has been a fierce debate in
the literature whether the sharp change in the spectrum near 18 Å is due to an
absorption edge (lower flux to the left) or a broad emission line (higher flux to the
right). Here we only note that this course is meant to train you as a spectroscopist
so that you can judge such issues yourself.
For the proper calculation of an X-ray spectrum, one should consider three different
steps:
1. the determination of the ionisation balance
2. the determination of the emitted spectrum
3. possible absorption of the photons on their way towards Earth
However, before discussing these processes we will first give a short summary of
atomic structure and radiative transitions, as these are needed to understand the
basic processes.
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