396 research outputs found
The Evolutionary Unified Scheme. I. Quasars and Radio Galaxies in the Viewing Angle - Redshift Plane
We present a study of the distribution of quasars and radio galaxies in the
plane (viewing angle - redshift) in the framework of the evolutionary unified
scheme (Vagnetti et al. 1991). Results are presented for some illustrative
cases, including a distribution of the Lorentz factors, and appropriate
luminosity functions for quasars and their host galaxies. A cosmologically
increasing is found, in agreement with the previous paper. It is argued
that the appearence of sources as quasars or radio galaxies can depend on the
viewing angle and on the redshift, due to the balance between the beamed
component and the luminosity of the host galaxy. Within the assumptions of our
evolutionary unified scheme, we find that low-Gamma objects can be observed as
quasars mainly at z<~0.3, while a substantial fraction of the low-z radio
galaxies could consist of quasar-remnants.Comment: to appear in ApJ; 18 pages, uuencoded-compressed-tarred PostScript
file including figures; ROM2F/94/1
Variability-selected low luminosity AGNs in the SA57 and in the CDFS
Low Luminosity Active Galactic Nuclei (LLAGNs) are contaminated by the light
of their host galaxies, thus they cannot be detected by the usual colour
techniques. For this reason their evolution in cosmic time is poorly known.
Variability is a property shared by virtually all active galactic nuclei, and
it was adopted as a criterion to select them using multi epoch surveys. Here we
report on two variability surveys in different sky areas, the Selected Area 57
and the Chandra Deep Field South.Comment: to appear in the proceedings of "The Starburst-AGN Connection
Conference", Shanghai, China, 27 Oct - 1 Nov 2008, ASP Conference Serie
Ensemble spectral variability study of Active Galactic Nuclei from the XMM-Newton serendipitous source catalogue
The variability of the X-ray spectra of active galactic nuclei (AGN) usually
includes a change of the spectral slope. This has been investigated for a small
sample of local AGNs by Sobolewska and Papadakis, who found that slope
variations are well correlated with flux variations, and that spectra are
typically steeper in the bright phase (softer when brighter behaviour). Not
much information is available for the spectral variability of high-luminosity
AGNs and quasars. In order to investigate this phenomenon, we use data from the
XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X-ray
observations for a large number of active galactic nuclei in a wide luminosity
and redshift range, for several different epochs. This allows to perform an
ensemble analysis of the spectral variability for a large sample of quasars. We
quantify the spectral variability through the spectral variability parameter
, defined as the ratio between the change in spectral slope and the
corresponding logarithmic flux variation. We find that the spectral variability
of quasars has a softer when brighter behaviour, similarly to local AGNs.Comment: 5 pages, 2 figures. Prepared for the proceedings of the 6th Young
Researchers Meeting, L'Aquil
Selecting AGN through variability in SN datasets
Variability is a main property of active galactic nuclei (AGN) and it was
adopted as a selection criterion using multi epoch surveys conducted for the
detection of supernovae (SNe). We have used two SN datasets. First we selected
the AXAF field of the STRESS project, centered in the Chandra Deep Field South
where, besides the deep X-ray surveys also various optical catalogs exist. Our
method yielded 132 variable AGN candidates. We then extended our method
including the dataset of the ESSENCE project that has been active for 6 years,
producing high quality light curves in the R and I bands. We obtained a sample
of ~4800 variable sources, down to R=22, in the whole 12 deg^2 ESSENCE field.
Among them, a subsample of ~500 high priority AGN candidates was created using
as secondary criterion the shape of the structure function. In a pilot
spectroscopic run we have confirmed the AGN nature for nearly all of our
candidates.Comment: 6 pages, 3 figures, contributed talk, proceedings of the 9th Hellenic
Astronomical Society Conference, Athens, 20-24 September 200
The X-ray/UV ratio in Active Galactic Nuclei: dispersion and variability
The relation between the index and the optical/UV luminosity
(), a by product of the X-ray - optical/UV luminosity relation, is
affected by a relatively large dispersion, due to variability in the
within single sources (intra-source dispersion) and variations of
fundamental physical parameters from source to source (inter-source
dispersion). We use archival data from the XMMSSC and from the XMMOM-SUSS3. We
select a sub-sample in order to decrease the dispersion of the relation due to
the presence of Radio-Loud and Broad Absorption Line objects, and to
absorptions in both X-ray and optical/UV bands. We analyse the dependence of
the residuals of the relation on various physical parameters in order to
characterise the inter-source dispersion. We find a total dispersion of ~0.12
and, using the Structure Function, we find that intrinsic variability
contributes for 56% of the variance of the relation. We find weak but
significant dependences of the residuals of the relation on black-hole (BH)
mass and on Eddington ratio, confirmed by a multivariate regression analysis of
as a function of optical/UV luminosity and the above quantities.
We find a weak positive correlation of both the and the residuals
of the relation with inclination indicators (FWHM(H) and EW[O])
suggesting a weak increase of X-ray/UV ratio with the viewing angle. Our
results suggest the possibility of selecting a sample of objects, based on
their viewing angle and/or BH mass and Eddington ratio, for which the
relation is as tight as possible, in light of the use of
the X-ray - optical/UV luminosity relation to build a distance modulus (DM) -
plane and estimate cosmological parameters.Comment: accepted for publication in A&
Variability and the X-ray/UV ratio of Active Galactic Nuclei
The observed relation between the X-ray radiation from AGNs, originating in
the corona, and the optical/UV radiation from the disk is usually described by
the anticorrelation between the UV to X-ray slope alpha_ox and the UV
luminosity. Many factors can affect this relation, including: enhanced X-ray
emission associated with the jets of radio-loud AGNs; X-ray absorption
associated with the UV Broad Absorption Line (BAL) outflows; other X-ray
absorption not associated with BALs; intrinsic X-ray weakness; UV and X-ray
variability, and non-simultaneity of UV and X-ray observations. The separation
of these effects provides information about the intrinsic alpha_ox-L_UV
relation and its dispersion, constraining models of disk-corona coupling. We
extract simultaneous data from the second XMM-Newton serendipitous source
catalogue and the XMM-Newton Optical Monitor Serendipitous UV Source Survey
Catalog, and derive the single-epoch alpha_ox indices. We use ensemble
structure functions to analyse multi-epoch data. We confirm the anticorrelation
of alpha_ox with L_UV, and do not find any evidence of a dependence of alpha_ox
on z. The dispersion in our simultaneous data (0.12) is not significantly
smaller than in previous non-simultaneous studies, suggesting that "artificial
alpha_ox variability" introduced by non-simultaneity is not the main cause of
dispersion. "Intrinsic alpha_ox variability", i.e., the true variability of the
X-ray to optical ratio, is instead important, and accounts for ~30% of the
total variance, or more. "Inter-source dispersion", due to intrinsic
differences in the average alpha_ox values from source to source, is also
important. The dispersion introduced by variability is mostly caused by the
long timescale variations, which are expected to be driven by the optical
variations.Comment: 16 pages, 10 figures, 1 table. Final version equal to the published
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