pdf - SRON

CHAPTER 1: INTRODUCTION
optically classified as Type-2 AGN (see Figure 1.4).
Unification schemes provide a simple description of the most general properties
of AGN. X-ray observations show that Type-2 AGN are systematically weaker
in the soft X-ray band than Type-1 AGN, as expected if their nuclear emission
is obscured. However, the number of observational results that cannot be explained
by the standard unified model are increasing rapidly. X-ray surveys are
finding that not all X-ray obscured AGN have Type-2 optical spectra, and that
not all Type-1 AGN are X-ray unobscured (e.g. [149], [150]).
1.6 Evolution of AGN in X-rays
1.6.1 The K-correction
The luminosities of extragalactic objects such as AGN have to be corrected for
their redshift, so that all luminosities are studied over the same emitted band
(we would have different emitted bands for different redshifts otherwise).
Formally, this can be expressed as:
L = L obs × K corr (z) (1.6.1)
where L obs is the luminosity in the observer bandpass and L is the luminosity
over the same bandpass in the frame of reference of the emitting object. For an
object with a power law spectrum (typical of AGN), the K-correction is:
K corr (z) = (1 + z) α−1 (1.6.2)
where α is the energy index defined in section 1.5.1. The K-correction is unity
(and therefore has no effect at all) when α = 1. In any other cases, neglecting
the K-correction would lead to spurious luminosity evolution with redshift.
In this work, the spectral index of most AGN is known and therefore we have
applied the K-correction for individual AGN. However, in some cases in which
we do not know the spectral index for every single source in a sample we have
applied an average K-correction.
21