Thermal X-ray radiation (PDF) - SRON

Figure 3.26: Spectrum of a plasma in collisional ionisation equilibrium with kT =
2 keV, near the Fe-K complex. Lines are labelled using the most common designations
in this field. The Fexxv ”triplet” consists of the resonance line (w),
intercombination line (actually split into x and y) and the forbidden line (z). All
other lines are satellite lines. The labelled satellites are lines from Fexxiv, most of
the lines with energy below the forbidden (z) line are from Fexxiii. The relative
intensity of these satellites is a strong indicator for the physical conditions in the
source.
the excited state may also decay to other intermediate states if present, and also
excitations or cascades from other levels may play a role. Furthermore, for high densities
also collisional excitation or de-excitation to and from other levels becomes
important. In general, one has to solve a set of equations for all energy levels of
an ion where all relevant population and depopulation processes for that level are
taken into account. For the resulting solution vector n j , the emitted line power is
then simply given by Eqn. (3.78).
Note that not all possible transitions between the different energy levels are
allowed. There are strict quantum mechanical selection rules that govern which
lines are allowed; see for instance Herzberg (1944) or Mee (1999). Sometimes there
are higher order processes that still allow a forbidden transition to occur, albeit with
much smaller transition probabilities A ij . But if the excited state j has no other
(fast enough) way to decay, these forbidden lines occur and the lines can be quite
strong, as their line power is essentially governed by the rate at which the ion is
brought into its excited state j.
One of the most well known groups of lines is the He-like 1s–2p triplet. Usually
the strongest line is the resonance line, an allowed transition. The forbidden line
has a similar strength as the resonance line, for the low density conditions in the
ISM and intracluster medium, but it can be relatively enhanced in recombining
plasmas, or relatively reduced in high density plasmas like stellar coronal loops. In
between both lines is the intercombination line. In fact, this intercombination line
is a doublet but for the lighter elements both components cannot be resolved. But
see Fig. 3.26 for the case of iron.
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