Thermal X-ray radiation (PDF) - SRON

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Chapter 3 Thermal X-ray radiation 1
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. 2
Page 1: High Energy Astrophysics Jelle Kaas
Page 5 and 6: Figure 3.2: The observed X-ray spec
Page 7 and 8: Figure 3.6: X-ray spectrum of the S
Page 9 and 10: series of alkaline spectra). The no
Page 11 and 12: • The first atomic shell (n = 1)
Page 13 and 14: Z El 1s 2s 2p 3s 3p 3d 4s Ground te
Page 15 and 16: m l1 s 1 m l2 s 2 M L M S +1 + +1
Page 17 and 18: 3.3 Energy levels We have seen the
Page 19 and 20: 3.4 Transitions between different l
Page 21 and 22: Figure 3.10: Transitions between a
Page 23 and 24: 3.5 Abundances of the elements With
Page 25 and 26: 3.6 Basic processes In order to be
Page 27 and 28: leading to Here the constant S 0 is
Page 29: 3.6.3 Radiative transition probabil
Page 32 and 33: Figure 3.12: Fluorescence yield as
Page 34 and 35: The formula of Younger While the Lo
Page 36 and 37: 3.6.6 Excitation-Autoionisation In
Page 38 and 39: 3.6.7 Photoionisation Figure 3.20:
Page 40 and 41: 3.6.9 Radiative recombination Radia
Page 42 and 43: 3.6.10 Dielectronic recombination T
Page 44 and 45: 3.7 The ionisation balance In order
Page 46 and 47: of computing time. Therefore one us
Page 48 and 49: 3.8 Continuum emission processes Af
Page 50 and 51: decays back to the 1s orbit by a ra
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Inner-shell ionisation The third li
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3.9.3 Line width For most X-ray spe
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Table 3.6: The strongest emission l
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Table 3.8: Some important iron line
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with τ(λ) = τ 0 ϕ(λ) (3.83) wh
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Table 3.9: The most important X-ray
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where the hydrogen column density N
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source itself. Fortunately, with hi
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Figure 3.35: Stability for the ioni
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Table 3.10: Wavelengths in Å of th
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