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Scientific Report 2007-2009
Astronomy & Astrophysics
A9. Spectral evolution and variability of Active Galactic Nuclei
The general term ’active galactic nuclei’ (AGNs) refers
to the existence, in the central region of some galaxies,
of energetic phenomena which cannot be attributed directly
to stars. The spectral energy distribution of these
sources, extends with comparable intensities from the
Radio up to the Gamma ray band, implying that several
physical mechanisms are involved, at variance with
stars where the bulk of the emission comes from thermal
black-body radiation. Their emission has been found to
account for nearly the whole cosmic X-ray background
radiation and their contribution is not negligible even
for the cosmic Microwave background (CMB). The basic
structure of an AGN is supposed to be a super-massive
black hole (SMBH) (from 10 6 up to 10 9 solar masses)
accreting matter from a surrounding disk: the conversion
of gravitational potential energy into electromagnetic
radiation powers the AGN emission. The mass of
the central black hole is correlated with the mass of the
host galaxy, indicating a physical connection between
the processes of galaxy and AGN formation, which is
one of the main subject of the present astrophysical research.
While AGNs are present in about 1 % of all
galaxies, most, if not all, galaxies are believed to host
in their nucleus a SMBH with very low or null energetic
activity for the absence of accretion processes. Most
AGN show variability at all wave-lengths and time scales
ranging from hours to years, which allow to investigate
their internal structure. A small fraction of AGN, called
Blazars, show particularly strong and rapid variability
and are polarized. These properties are related with
the presence of two opposite jets of material escaping
at relativistic speed from the central region. The main
emission processes in Blazars are a synchrotron component,
due to the relativistic electron moving in the magnetic
field of the jet, and an inverse Compton component,
most likely due to interaction of the same electrons on
the synchrotron photons, or to external photons. The
two processes peak respectively at optical and X-ray frequencies,
so that multi-wavelength, and therefore multiinstrument,
observations must be simultaneously made
to measure both components.
Since several years, our group is involved in these multiwavelength
campaigns, which often imply large international
collaborations using both ground based (optical,
radio, TeV) and space based (X-ray, Gamma-ray)
instruments: a recent example paper of this kind is reported
below [1]. We are also involved in the study of
the long term optical variability of Blazars, using our
telescope at Vallinfreda and archive photographic plates
from the Asiago Observatory [2]. The technique of digitization
and data analysis has been mainly set up in
the years 2002/04 by our group in the framework of a
National Project.
Statistical samples of AGN, mostly quasars (QSOs)
and Seyfert galaxies, can be detected through their
Figure 1: The time dependent spectral energy distribution of
the Blazar 3C 454.3 resulting from multi-epoch observations,
from the radio to the Gamma-ray band [1].
variability. This makes possible the detection of faint
AGNs, which cannot be selected on the basis of their
colours, since they are affected by the light of the host
galaxy. We created and analysed various samples of this
type. For the sample of the Selected Area 57, observed
in the optical band for more than 15 years at the Kitt
Peak National Observatory to identify variable sources,
we obtained observing time with the XMM-Newton
X-ray Observatory [3]. Another sample of this type was
created in the Chandra Deep Field South, where the
deepest X-ray observations (2 Ms) exist. A third sample,
which is one of the largest ever detected on the sole
basis of variability, was obtained from the optical data
collected by the ESSENCE international collaboration,
which is devoted to the measure of the cosmological
parameters through the analysis of deep supernova
samples [4]. Variability-selected samples make possible
a combined X-ray and optical analysis. The AGN
nature of several variability-detected candidates has
been confirmed by X-ray emission. We discovered some
objects, whose AGN nature has been confirmed by
optical spectroscopy, which are not detected in X-ray
due to their particularly low X-ray to optical ratio.
References
1. Raiteri et al. A&A 491, 755, (2008)
2. Nesci R., et al., AJ 133, 965, (2007)
3. Trevese, D., et al. A&A 469, 1211, (2007)
4. Boutsia, K., et al. A&A 497, 81, (2009)
Authors
K. Boutsia, S. Gaudenzi, R. Nesci, S. Piranomonte, M.
Tomei, D. Trevese
http://astrowww.phys.uniroma1.it/scae.html
Sapienza Università di Roma 156 Dipartimento di Fisica