and Cosmology

5. Active Galactic Nuclei
200
Fig. 5.23. The different
light curves from Fig. 5.22
are correlated with the continuum
flux at λ = 1350 Å.
The autocorrelation function
is shown by the
solid line in the central
panels, the others are crosscorrelation
functions. We
can see that the maximum
of the correlation is shifted
towards positive times –
variations in the continuum
flux are not simultaneously
followed by the emission
lines but appear only after
a delay. This delay corresponds
to the light travel
time from the center of the
AGN to the clouds of the
BLR where the lines are
emitted. The smaller the
ionization level of the respective
ion, the longer the
delay. For example, we obtain
a delay of 12 days for
Lyα, 26daysforCIII], and
about 50 days for MgII,
where the latter value could
not be measured exactly
because the relative flux
variations of this line are
small and thus the correlation
function does not
show a very prominent
maximum
only plausible one, an outflow motion of the clouds in
the BLR can be assumed, where we see those clouds
that are, from our point of view, located behind the accretion
disk partly absorbed by the disk material. The
received line radiation is therefore dominated by the
clouds that are in front of the disk and moving towards
us, so that it is systematically blueshifted.
The nature of the clouds in the BLR is unknown.
Their small extent and high temperature imply that they
should vaporize on very small time-scales unless they
are somehow stabilized. Therefore these clouds need to
be either permanently replenished or they have to be
stabilized, either by external pressure, e.g., from a very
hot but thin medium in the BLR in between the clouds,