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