Thermal X-ray radiation (PDF) - SRON

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source itself. Fortunately, with high spectral resolution the cosmologically redshifted absorption lines from the X-ray source are separated well from the foreground hot ISM absorption lines. Only for lines from the Local Group this is not possible, and the situation is here complicated as the expected temperature range for the diffuse gas within the Local Group is similar to the temperature of the hot ISM. Another ISM component is dust. A significant fraction of some atoms can be bound in dust grains with varying sizes, as shown below (from Wilms et al. 2000): H He C N O Ne Mg Si S Ar Ca Fe Ni 0 0 0.5 0 0.4 0 0.8 0.9 0.4 0 0.997 0.7 0.96 The numbers represent the fraction of the atoms that are bound in dust grains. Noble gases like Ne and Ar are chemically inert hence are generally not bound in dust grains, but other elements like Ca exist predominantly in dust grains. Dust has significantly different X-ray properties compared to gas or hot plasma. First, due to the chemical binding, energy levels are broadened significantly or even absent. For example, for oxygen in most bound forms (like H 2 O) the remaining two vacancies in the 2p shell are effectively filled by the two electrons from the other bound atom(s) in the molecule. Therefore, the strong 1s–2p absorption line at 23.51 Å (527 eV) is not allowed when the oxygen is bound in dust or molecules, because there is no vacancy in the 2p shell. Transitions to higher shells such as the 3p shell are possible, however, but these are blurred significantly and often shifted due to the interactions in the molecule. Each constituent has its own fine structure near the K-edge (Fig. 3.31b). This fine structure offers therefore the opportunity to study the (true) chemical composition of the dust, but it should be said that the details of the edges in different important compounds are not always (accurately) known, and can differ depending on the state: for example water, crystalline and amorphous ice all have different characteristics. On the other hand, the large scale edge structure, in particular when observed at low spectral resolution, is not so much affected. For sufficiently high column densities of dust, self-shielding within the grains should be taken into account, and this reduces the average opacity per atom. Finally, we mention here that dust also causes scattering of X-rays. This is in particular important for higher column densities. For example, for the Crab nebula (N H = 3.2×10 25 m −2 ), at an energy of 1 keV about 10 % of all photons are scattered in a halo, of which the widest tails have been observed out to a radius of at least half a degree; the scattered fraction increases with increasing wavelength. 3.10.7 Absorption from ionised gas The absorption properties become different when the plasma is ionised. Fig. 3.32 gives the effective cross section for plasmas in CIE. For 10 4 K, σ is essentially at the neutral limit. For 2 × 10 4 K hydrogen is fully ionised, and therefore σ for energies of a few hundred eV is decreasing; at 10 5 K also He is ionised and σ is in particular small below the C-edge. Around the O-edge (the region between 0.5–1 keV) we see that by increasing the ionisation of oxygen the edge shifts from 533 eV (Oi) to 870 eV (Oviii). Below the O-edge σ becomes very small for increasing temperatures, because the most important absorbers (H, He) are fully ionised. But unless oxygen is fully ionised (3 × 10 6 K) σ above the oxygen K-edge is decreasing only slowly. Because of this the contrast around the K-edge of 65
Figure 3.32: The optical depth per hydrogen atom times E 3 for a plasma in CIE. Figure 3.33: The optical depth per hydrogen atom times E 3 for a plasma in PIE. oxygen increases for increasing T. Similar features are seen around the K-edge of Si (∼ 2 keV), S (∼ 2.5 keV) and in particular Fe (7–8.5 keV) between 10 5 K and 3 × 10 8 K. Therefore we see that for T > 10 7 K the plasma is almost transparent below 2 keV. between 10 7 and 10 8 K, σ between 2-6 keV is an order of magnitude smaller than for neutral plasma’s, while above the K-edge of iron the effective cross section hardly changes, unless T > 10 8 K. For plasmas in photoionisation one finds qualitatively a similar behaviour (Fig. 3.33). The parameter Ξ is the so-called ionisation parameter: Ξ ≡ L/4πcr 2 n H kT. (3.91) Essentially, it represents the ratio between radiation pressure and gas pressure. In general, Ξ depends on the precise form of the incoming X-ray spectrum. For a power law spectrum with energy index 1, or photon index 2 (dF/dE ∼ E −1 ) the equilibrium solution for the temperature as a function of Ξ is shown in Fig. 3.34. 66
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High Energy Astrophysics Jelle Kaas
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3.1 Introduction Thermal X-ray radi
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Figure 3.2: The observed X-ray spec
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Figure 3.6: X-ray spectrum of the S
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series of alkaline spectra). The no
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• The first atomic shell (n = 1)
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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
Page 52 and 53: Inner-shell ionisation The third li
Page 54 and 55: 3.9.3 Line width For most X-ray spe
Page 56 and 57: Table 3.6: The strongest emission l
Page 58 and 59: Table 3.8: Some important iron line
Page 60 and 61: with τ(λ) = τ 0 ϕ(λ) (3.83) wh
Page 62 and 63: Table 3.9: The most important X-ray
Page 64 and 65: where the hydrogen column density N
Page 68 and 69: Figure 3.35: Stability for the ioni
Page 70: Table 3.10: Wavelengths in Å of th
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