163 research outputs found
The nature of gas and stars in the circumnuclear regions of AGN: a chemical approach
Aim of this communication is to describe the first results of a
work-in-progress regarding the chemical properties of gas and stars in the
circumnuclear regions of nearby galaxies. Different techniques have been
employed to estimate the abundances of chemical elements in the gaseous and
stellar components of nuclear surroundings in different classes of galaxies
according to the level of activity of the nucleus (normal or passive, star
forming galaxies and AGNs).Comment: 19 pages, proceedings of the 1st International Workshop:
Astrophysical winds and disk 2009 (Platamonas
V4743 Sgr, a magnetic nova?
Two XMM Newton observations of Nova V4743 Sgr (Nova Sgr 2002) were performed
shortly after it returned to quiescence, 2 and 3.5 years after the explosion.
The X-ray light curves revealed a modulation with a frequency of ~0.75 mHz,
indicating that V4743 Sgr is most probably an intermediate polar (IP). The
X-ray spectra have characteristics in common with known IPs, with a hard
thermal plasma component that can be fitted only assuming a partially covering
absorber. In 2004 the X-ray spectrum had also a supersoft blackbody-like
component, whose temperature was close to that of the white dwarf (WD) in the
supersoft X-ray phase following the outburst, but with flux by at least two
orders of magnitude lower. In quiescent IPs, a soft X-ray flux component
originates at times in the polar regions irradiated by an accretion column, but
the supersoft component of V4743 Sgr disappeared in 2006, indicating a possible
origin different from accretion. We suggest that it may have been due to an
atmospheric temperature gradient on the WD surface, or to continuing localized
thermonuclear burning at the bottom of the envelope, before complete turn-off.
An optical spectrum obtained with SALT 11.5 years after the outburst showed a
prominent He II 4686A line and the Bowen blend, which reveal a very hot region,
but with peak temperature shifted to the ultraviolet (UV) range. V4743 Sgr is
the third post-outburst nova and IP candidate showing a low-luminosity
supersoft component in the X-ray flux a few years after the outburst.Comment: 9 pages, 5 figures, accepted to MNRA
Spectral properties of the narrow-line region in Seyfert galaxies selected from the SDSS-DR7
Although the properties of the narrow-line region (NLR) of active galactic
nuclei(AGN) have been deeply studied by many authors in the past three decades,
many questions are still open. The main goal of this work is to explore the NLR
of Seyfert galaxies by collecting a large statistical spectroscopic sample of
Seyfert 2 and Intermediate-type Seyfert galaxies having a high signal-to-noise
ratio in order to take advantage of a high number of emission-lines to be
accurately measured. 2153 Seyfert 2 and 521 Intermediate-type Seyfert spectra
were selected from Sloan Digital Sky Survey - Data Release 7 (SDSS-DR7) with a
diagnostic diagram based on the oxygen emission-line ratios. All the
emission-lines, broad components included, were measured by means of a
self-developed code, after the subtraction of the stellar component. Physical
parameters, such as internal reddening, ionization parameter, temperature,
density, gas and stellar velocity dispersion were determined for each object.
Furthermore, we estimated mass and radius of the NLR, kinetic energy of the
ionized gas, and black-hole accretion rate. From the emission-line analysis and
the estimated physical properties, it appears that the NLR is similar in
Seyfert 2 and Intermediate-Seyfert galaxies. The only differences, lower
extinction, gas kinematics in general not dominated by the host galaxy
gravitational potential and higher percentage of [O III]5007 blue asymmetries
in Intermediate-Seyfert can be ascribed to an effect of inclination of our line
of sight with respect to the torus axis.Comment: Accepted for publication in MNRA
Optical Emission Lines and the X-Ray Properties of Type 1 Seyfert Galaxies
In this contribution we report on the study of the optical emission lines and
X-ray spectra of a sample of Type 1 AGNs, collected at the Sloan Digital Sky
Survey database and observed by the XMM Newton satellite. Exploiting the
different instruments carried onboard XMM, we identify the spectral components
of the soft and hard energy bands (in the range from 0.3 keV up to 10 keV). The
properties of the X-ray continuum and of the Fe Kalpha line feature are
investigated in relation to the optical broad emission line profiles and
intensity ratios. The resulting picture of emission, absorption and reflection
processes is interpreted by means of a BLR structural model that was developed
on the basis of independent optical and radio observations.Comment: 6 pages, 3 figures, Proceedings of the VIII Serbian Conference on
Spectral Line Shapes in Astrophysics, accepted for publication on Baltic
Astronomy. Corrected typos in V
Optical counterparts of undetermined type -ray Active Galactic Nuclei with blazar-like Spectral Energy Distributions
During its first four years of scientific observations, the Fermi Large Area
Telescope (Fermi-LAT) detected 3033 -ray sources above a 4
significance level. Although most of the extra-Galactic sources are active
galactic nuclei (AGN) of the blazar class, other families of AGNs are observed
too, while a still high fraction of detections () remains with
uncertain association or classification. According to the currently accepted
interpretation, the AGN -ray emission arises from inverse Compton (IC)
scattering of low energy photons by relativistic particles confined in a jet
that, in the case of blazars, is oriented very close to our line of sight.
Taking advantage of data from radio and X-ray wavelengths, which we expect to
be produced together with -rays, providing a much better source
localization potential, we focused our attention on a sample of -ray
Blazar Candidates of Undetermined Type (BCUs), starting a campaign of optical
spectroscopic observations. The main aims of our investigation include a census
of the AGN families that contribute to -ray emission and a study of
their redshift distribution, with the subsequent implications on the intrinsic
source power. We furthermore analyze which -ray properties can better
constrain the nature of the source, thus helping in the study of objects not
yet associated with a reliable low frequency counterpart. In this communication
we report on the instruments and techniques used to identify the optical
counterparts of -ray sources, we give an overview on the status of our
work, and we discuss the implications of a large scale study of -ray
emitting AGNs.Comment: 9 pages, 2 figures, proceedings of the 10th Serbian Conference on
Spectral Line Shapes in Astrophysics. JOAA, accepte
The Relation between Nuclear Activity and Stellar Mass in Galaxies
The existence of correlations between nuclear properties of galaxies, such as
the mass of their central black holes, and larger scale features, like the
bulge mass and luminosity, represent a fundamental constraint on galaxy
evolution. Although the actual reasons for these relations have not yet been
identified, it is widely believed that they could stem from a connection
between the processes that lead to black hole growth and stellar mass assembly.
The problem of understanding how the processes of nuclear activity and star
formation can affect each other became known to the literature as the
Starburst-AGN connection. Despite years of investigation, the physical
mechanisms which lie at the basis of this relation are known only in part. In
this work, we analyze the problem of star formation and nuclear activity in a
large sample of galaxies. We study the relations between the properties of the
nuclear environments and of their host galaxies. We find that the mass of the
stellar component within the galaxies of our sample is a critical parameter,
that we have to consider in an evolutionary sequence, which provides further
insight in the connection between AGN and star formation processes.Comment: 13 pages, 10 figures, accepted for publication on MNRAS. Reference to
the mass derivation procedure correcte
A spectroscopic analysis of a sample of narrow-line Seyfert 1 galaxies selected from the Sloan Digital Sky Survey
We revisited the spectroscopic characteristics of narrow-line Seyfert 1
galaxies (NLS1s) by analysing a homogeneous sample of 296 NLS1s at redshift
between 0.028 and 0.345, extracted from the Sloan Digital Sky Survey (SDSS-DR7)
public archive. We confirm that NLS1s are mostly characterized by Balmer lines
with Lorentzian profiles, lower black hole masses and higher Eddington ratios
than classic broad-line Seyfert 1 (BLS1s), but they also appear to be active
galactic nuclei (AGNs) contiguous with BLS1s and sharing with them common
properties. Strong Fe II emission does not seem to be a distinctive property of
NLS1s, as low values of Fe II/H are equally observed in these AGNs. Our
data indicate that Fe II and Ca II kinematics are consistent with the one of
H. On the contrary, O I 8446 seems to be systematically
narrower and it is likely emitted by gas of the broad-line region more distant
from the ionizing source and showing different physical properties. Finally,
almost all NLS1s of our sample show radial motions of the narrow-line region
highly-ionised gas. The mechanism responsible for this effect is not yet clear,
but there are hints that very fast outflows require high continuum luminosities
(> erg/s) or high Eddington ratios (log(L/L)
> -0.1).Comment: 27 pages, 31 figures, 4 tables, accepted for publication in MNRA
[O III] line properties in two samples of radio-emitting narrow-line Seyfert 1 galaxies
The [O III] 4959,5007 lines are a useful proxy to test the
kinematic of the narrow-line region (NLR) in active galactic nuclei (AGN). In
AGN, and particularly in narrow-line Seyfert 1 galaxies (NLS1s) these lines
often show few peculiar features, such as blue wings, often interpreted as
outflowing component, and a shift typically toward lower wavelengths of
the whole spectroscopic feature in some exceptional sources, the so-called blue
outliers, which are often associated to strong winds. We investigated the
incidence of these peculiarities in two samples of radio-emitting NLS1s, one
radio-loud and one radio-quiet. We also studied a few correlations between the
observational properties of the [O III] lines and those of the AGN. Our aim was
to understand the difference between radio-quiet and radio-loud NLS1s, which
may in turn provide useful information on the jet formation mechanism. We find
that the NLR gas is much more perturbed in radio-loud than in radio-quiet
NLS1s. In particular the NLR dynamics in -ray emitting NLS1s appears to
be highly disturbed, and this might be a consequence of interaction with the
relativistic jet. The less frequently perturbed NLR in radio-quiet NLS1s
suggests instead that these sources likely do not harbor a fully developed
relativistic jet. Nonetheless blue-outliers in radio-quiet NLS1s are observed,
and we interpret them as a product of strong winds.Comment: 16 pages, 13 figures, 7 tables, accepted for publication on Astronomy
& Astrophysic
Are Boltzmann Plots of Hydrogen Balmer lines a tool for identifying a subclass of S1 AGN?
It is becoming clear that we can define two different types of nearby AGN
belonging to the Seyfert 1 class (S1), on the basis of the match of the
intensities of their Broad Balmer Lines (BBL) with the Boltzmann Plots (BP).
These two types of S1 galaxies, that we call BP-S1 and NoBP-S1, are
characterized, in first approximation, by Broad Line Regions (BLR) with
different structural and physical properties. In this communication, we show
that these features can be well pointed out by a multi-wavelength analysis of
the continuum and of the broad recombination Hydrogen lines, that we carry out
on a sample of objects detected at optical and X-ray frequencies. The
investigation is addressed to verify whether BP-S1 are the ideal candidates for
the study of the kinematical and structural properties of the BLR, in order to
derive reliable estimates of the mass of their central engine and to constrain
the properties of their nuclear continuum spectrum.Comment: 26 pages, 4 figures (1 multi-paged). Accepted for publication on
Advances in Space Research. Contribution to the IX SCSLS
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