3.3. X-rays
Obscuration in the X-rays is due to photoelectric absorption (dominant below ~ 3 keV) and Compton scattering (dominant from ~ 7 to ~ 30 keV). The X-ray spectral properties of obscured AGN depend on the amount of absorbing column density: column densities below ~ 1.5 × 1024 cm-2 produce a photoelectric cut-off at energies between 1 and 10 keV (in this case, the source is "Compton thin"); column densities between ~ 1024 cm-2 and ~ 1025 cm-2 absorb the X-ray primary emission up to several tens of keV; even higher column densities completely obscure the central source in the X-rays.
In heavily absorbed sources ("Compton thick",
NH > 1024 cm-2),
the two main spectral features are a prominent
iron K
emission line
with EW ~ 1 - 3 keV,
and a reflected and/or scattered continuum.
In less obscured sources, the EW of the iron line depends on the fraction
of the intrinsic continuum emission absorbed at the line energy;
for NH < 1023 cm-2, values
typical of type I AGN are observed (EW ~ 100 - 300 eV), in agreement
with the unified model.
The reflected/scattered component is the same as described in
Section 2.4 for type I AGN.
In Figure 11, we plot the 1 - 100 keV spectra of four representative obscured AGN: MCG-5-23-16 (NH = 1022 cm-2, Risaliti 2002), NGC 4388 (NH = 4 × 1023 cm-2, Risaliti 2002), NGC 4945 (NH = 2 × 1024 cm-2, Vignati et al. 1999), and NGC 1068 (NH > 1025 cm-2, Matt et al. 1997).
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Figure 11. Four 2 - 100 keV BeppoSAX best fit X-ray spectra of Seyfert 2 galaxies. Main components of the best fit models are also shown. MCG-5-23-16 and NGC 4388 (Risaliti 2002) are "Compton thin", i.e., they are dominated by the primary emission down to a few keV. In MCG-5-23-16, a cold reflection component also gives a measurable contribution. The continuum in the Compton-thick source NGC 4945 (Guainazzi et al. 2000) is due to a warm reflection component in the 2 - 10 keV range, while at higher energies the intrinsic component emerges. Note the high ratio between the 10 - 100 keV and the 2 - 10 keV emission, as compared with the Compton-thin sources. NGC 1068, also Compton-thick (Matt et al. 1997), shows a cold reflection and a warm reflection component. Equivalent widths of the iron line are ~ 100 eV in MCG-5-23-16, ~ 500 eV in NGC 4388, and 1 - 2 keV in NGC 4945 and NGC 1068. |