Thermal X-ray radiation (PDF) - SRON

exercise 3.12. The hydrogen column density to the Galactic pole is about 10 24 m −2 .
How large is the transmission for photons of 0.1, 0.2, 1 and 10 keV?
Complexity of the ISM
Figure 3.31: Left panel: Simulated 100 ks absorption spectrum as observed with the
Explorer of Diffuse Emission and Gamma-ray Burst Explosions (EDGE), a mission
proposed for ESA’s Cosmic Vision program. The parameters of the simulated source
are similar to those of 4U 1820−303 (Yao et al. 2006). The plot shows the residuals
of the simulated spectrum if the absorption lines in the model are ignored. Several
characteristic absorption features of both neutral and ionised gas are indicated.
Right panel: Simulated spectrum for the X-ray binary 4U 1820−303 for 100 ks with
the WFS instrument of EDGE. The simulation was done for all absorbing oxygen in
its pure atomic state, the models plotted with different line styles show cases where
half of the oxygen is bound in CO, water ice or olivine. Note the effective shift of
the absorption edge and the different fine structure near the edge. All of this is well
resolved by EDGE, allowing a determination of the molecular composition of dust
in the line of sight towards this source. The absorption line at 0.574 is due to highly
ionised Ovii.
The interstellar medium is by no means a homogeneous, neutral, atomic gas. In
fact, it is a collection of regions all with different physical state and composition.
This affects its X-ray opacity. We briefly discuss here some of the most important
features.
The ISM contains cold gas (< 50 K), warm, neutral or lowly ionised gas (6000
− 10000 K) as well as hotter gas (a few million K). We have seen before that for
ions the absorption edges shift to higher energies for higher ionisation. Thus, with
instruments of sufficient spectral resolution, the degree of ionisation of the ISM can
be deduced from the relative intensities of the absorption edges. Cases with such
high column densities are rare, however (and only occur in some AGN outflows), and
for the bulk of the ISM the column density of the ionised ISM is low enough that only
the narrow absorption lines are visible (see Fig. 3.27). These lines are only visible
when high spectral resolution is used (see Fig. 3.31). It is important to recognise
these lines, as they should not be confused with absorption line from within the
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