79 research outputs found
Relativistic plasmas in AGN jets - From synchrotron radiation to -ray emission
Relativistic jets of plasma are a key ingredient of many types of Active
Galactic Nuclei (AGN). Today we know that AGNs are powered by the accretion of
inter stellar material into the gravitational field of a Super Massive Black
Hole and that this process can release as much power as a whole galaxy, like
the Milky Way, from a region that is comparable to the Solar System in size.
Depending on the properties of the central energy source, a large fraction of
this power can be involved in the acceleration of magnetized plasmas at
relativistic speeds, to form large scale jets. The presence of jets affects the
spectrum of AGNs through the emission of synchrotron radiation and Inverse
Compton scattering of low energy photons, thus leading to a prominent
non-thermal spectrum, some times extending from radio frequencies all the way
up to -ray energies. Here we review some characteristic processes of
radiation emission in AGN jets, which lead to the emission of photons in the
radio, optical, X-ray and -ray bands, and we present the results of a
spectroscopic campaign of optical counterparts. We discuss our observations and
their connection with -ray properties in a scenario that traces the
role of relativistic jets in different classes of AGNs, detected both in the
local as well as in the remote Universe.Comment: 11 pages, 5 figures (3 in color), proceedings of the XXVIII School on
Physics of Ionized Gases (SPIG), accepted for publication on the European
Journal of Physics
X-Ray Grating Observations of Recurrent Nova T Pyxidis During The 2011 Outburst
The recurrent nova T Pyx was observed with the X-ray gratings of Chandra and
XMM-Newton, 210 and 235 days, respectively, after the discovery of the 2011
April 14 outburst. The X-ray spectra show prominent emission lines of C, N, and
O, with broadening corresponding to a full width at half maximum of ~2000-3000
km/s, and line ratios consistent with high-density plasma in collisional
ionization equilibrium. On day 210 we also measured soft X-ray continuum
emission that appears to be consistent with a white dwarf (WD) atmosphere at a
temperature ~420,000 K, partially obscured by anisotropic, optically thick
ejecta. The X-ray continuum emission is modulated with the photometric and
spectroscopic period observed in quiescence. The continuum at day 235 indicated
a WD atmosphere at a consistent effective temperature of 25 days earlier, but
with a lower flux. The effective temperature indicates a mass of ~1 solar mass.
The conclusion of partial WD obscuration is supported by the complex geometry
of non-spherically-symmetric ejecta confirmed in recent optical spectra
obtained with the Southern African Large Telescope (SALT) in November and
December of 2012. These spectra exhibited prominent [O III] nebular lines with
velocity structures typical of bipolar ejecta.Comment: Accepted to ApJ 2013 October 23, 14 pages, 9 figures, 3 table
Jetted narrow-line Seyfert 1 galaxies breaking the jet paradigm -- a comprehensive study of host galaxy morphologies
Narrow-line Seyfert 1 (NLS1) galaxies are unevolved active galactic nuclei
(AGN) that exist predominantly in spiral galaxies. However, mostly due to the
small number of sources studied, it has been under debate whether also the
hosts of jetted NLS1 galaxies, a particular subclass of these sources hosting a
relativistic jet, are disk-like, or elliptical, as the hosts of more powerful
jetted AGN. We studied the host morphologies of 14 NLS1 galaxies, 11 of which
have been detected at 37 GHz indicating that these sources harbour relativistic
jets. The J- and Ks-band data used in this study were obtained with the Nordic
Optical Telescope (NOT). We performed the photometric decomposition of the host
galaxy using the band that gave a better fit, and additionally, created colour
maps of all sources that had both a J- and a Ks-band observation. We were able
to successfully model 12 sources, nine of which most likely have disk-like
morphology. Of the remaining sources, one source could possibly be hosted
either in a disk-like or a dwarf galaxy, and in two cases the results are
inconclusive. Only one of our sources shows clear signs of interaction, but the
colour maps of most of our sources hint at ample dust in the nuclei, possibly
indicating earlier minor mergers, that can go unnoticed due to the limited
resolution of these observations. Our results further support disk-like
galaxies as the predominant host type of jetted NLS1 galaxies. Most
importantly, with the number of modelled hosts of jetted NLS1s now exceeding
50, with only a few elliptical hosts, it seems to be safe to conclude that also
disk-like galaxies are able to launch and maintain relativistic jets, and that
the traditional jet paradigm stating that only massive elliptical galaxies are
capable of hosting relativistic jets is severely outdated.Comment: 24 pages, 40 figures, 17 tables, accepted to A&
Models of emission line profiles and spectral energy distributions to characterize the multi-frequency properties of active galactic nuclei
The spectra of Active Galactic Nuclei (AGNs) are often characterized by a
wealth of emission lines with different profiles and intensity ratios that led
to a complicated classification. Their electro-magnetic radiation spans more
than 10 orders of magnitude in frequency. In spite of the differences between
various classes, the origin of their activity is attributed to a combination of
emitting components, surrounding an accreting Super Massive Black Hole, in the
so called Unified Model. Currently, the execution of sky surveys, with
instruments operating at various frequencies, provides the possibility to
detect and to investigate the properties of AGNs on very large statistical
samples. Thanks to the spectroscopic surveys that allow investigation of many
objects, we have the opportunity to place new constraints on the nature and
evolution of AGNs. In this contribution we present the results obtained by
working on multi-frequency data and we discuss their relations with the
available optical spectra. We compare our findings with the AGN Unified Model
predictions, and we present a revised technique to select AGNs of different
types from other line emitting objects. We discuss the multi-frequency
properties in terms of the innermost structures of the sources.Comment: 11 pages, 4 figures. Proceedings of the XI Serbian Conference on
Spectral Line Shapes in Astrophysics. Accepted for publication on Atom
An orientation-based unification of young jetted AGN: the case of 3C 286
In recent years, the old paradigm according to which only high-mass black
holes can launch powerful relativistic jets in active galactic nuclei (AGN) has
begun to crumble. The discovery of -rays coming from narrow-line
Seyfert 1 galaxies (NLS1s), usually considered young and growing AGN harboring
a central black hole with mass typically lower than 10 M, indicated
that also these low-mass AGN can produce powerful relativistic jets. The search
for parent population of -ray emitting NLS1s revealed their connection
with compact steep-spectrum sources (CSS). In this proceeding we present a
review of the current knowledge of these sources, we present the new important
case of 3C 286, classified here for the fist time as NLS1, and we finally
provide a tentative orientation based unification of NLS1s and CSS sources.Comment: 12 pages, 3 figures. Proceeding of the conference "Quasars at all
cosmic epochs", held in Padova, April 2-7, 2017, published on Frontiers in
Astronomy and Space Science
Parent population of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies
Flat-spectrum radio-loud Narrow-Line Seyfert 1 galaxies (NLS1s) are a
recently discovered class of -ray emitting Active Galactic Nuclei
(AGN), that exhibit some blazar-like properties which are explained with the
presence of a relativistic jet viewed at small angles. When blazars are
observed at larger angles they appear as radio-galaxies, and we expect to
observe an analogue parent population for beamed NLS1s. However, the number of
known NLS1s with the jet viewed at large angles is not enough. Therefore, we
tried to understand the origin of this deficit. Current hypotheses about the
nature of parent sources are steep-spectrum radio-loud NLS1s, radio-quiet NLS1s
and disk-hosted radio-galaxies. To test these hypotheses we built three samples
of candidate sources plus a control sample, and calculated their black hole
mass and Eddington ratio using their optical spectra. We then performed a
Kolmogorov-Smirnov statistical test to investigate the compatibility of our
different samples with a beamed population. Our results indicate that, when the
inclination angle increases, a beamed source appears as a steep-spectrum
radio-loud NLS1, or possibly even as a disk-hosted radio-galaxy with low black
hole mass and high Eddington ratio. Further investigations, involving larger
complete samples and observations at radio frequency, are needed to understand
the incidence of disk-hosted radio-galaxies in the parent population, and to
assess whether radio-quiet NLS1s can play a role, as well.Comment: 12 pages, 6 figures, accepted for publication by Astronomy and
Astrophysic
Extended narrow-line region in Seyfert galaxies
We present our recent results about the extended narrow-line region (ENLR) of
two nearby Seyfert 2 galaxies (IC 5063 and NGC 7212) obtained by modelling the
observed line profiles and spectra with composite models
(photoionization+shocks) in the different regions surrounding the AGN. Then, we
compare the Seyfert 2 ENLRs with the very extended one recently discovered in
the narrow-line Seyfert 1 (NLS1) galaxy Mrk 783. We have found several
evidences of interaction between the ISM of the galaxies and their radio jets,
such as a) the contribution of shocks in ionizing the high velocity gas, b) the
complex kinematics showed by the profile of the emission lines, c) the high
fragmentation of matter, etc. The results suggest that the ENLR of IC 5063 have
a hollow bi-conical shape, with one edge aligned to the galaxy disk, which may
cause some kind of dependence on velocity of the ionization parameter.
Regarding the Mrk 783 properties, it is found that the extension of the optical
emission is almost twice the size of the radio one and it seems due to the AGN
activity, although there is contamination by star formation around 12 arcsec
from the nucleus. Diagnostic diagrams excluded the contribution of star
formation in IC 5063 and NGC 7212, while the shock contribution was used to
explain the spectra emitted by their high velocity gas.Comment: 9 pages, 2 figures, proceeding of the conference "Quasars at all
cosmic epochs", accepted for publication in Front. Astron. Space Sci. - Milky
Way and Galaxie
Signatures of an eruptive phase before the explosion of the peculiar core-collapse SN 2013gc
We present photometric and spectroscopic analysis of the peculiar
core-collapse SN 2013gc, spanning seven years of observations. The light curve
shows an early maximum followed by a fast decline and a phase of almost
constant luminosity. At +200 days from maximum, a brightening of 1 mag is
observed in all bands, followed by a steep linear luminosity decline after +300
d. In archival images taken between 1.5 and 2.5 years before the explosion, a
weak source is visible at the supernova location, with mag20. The
early supernova spectra show Balmer lines, with a narrow (560 km
s) P-Cygni absorption superimposed on a broad (3400 km s)
component, typical of type IIn events. Through a comparison of colour curves,
absolute light curves and spectra of SN 2013gc with a sample of supernovae IIn,
we conclude that SN 2013gc is a member of the so-called type IId subgroup. The
complex profile of the H line suggests a composite circumstellar medium
geometry, with a combination of lower velocity, spherically symmetric gas and a
more rapidly expanding bilobed feature. This circumstellar medium distribution
has been likely formed through major mass-loss events, that we directly
observed from 3 years before the explosion. The modest luminosity
( near maximum) of SN 2013gc at all phases, the very small amount
of ejected Ni (of the order of M), the major
pre-supernova stellar activity and the lack of prominent [O I] lines in
late-time spectra support a fall-back core-collapse scenario for the massive
progenitor of SN~2013gc.Comment: 20 pages, 11 figures, 8 tables, accepted by MNRA
Optical properties of two complementary samples of intermediate Seyfert galaxies
We present preliminary results of the analysis of optical spectra of two
complementary samples of Seyfert galaxies. The first sample was extracted from
a selection of the 4th Fermi Gamma-ray Large Area Telescope (4FGL) catalog, and
consists of 9 -ray emitting jetted Seyfert galaxies. The second one was
extracted from the Swift-BAT AGN Spectroscopic Survey (BASS), and is composed
of 38 hard-X ray selected Active Galactic Nuclei (AGN). These two samples are
complementary, with the former expected to have smaller viewing angles, while
the latter may include objects with larger viewing angles. We measured emission
line ratios to investigate whether the behavior of these Seyferts can be
explained in terms of obscuration, as suggested by the well-known Unified Model
(UM) of AGN, or if there are intrinsic differences due to the presence of jets,
outflows, or the evolution. We found no indications of intrinsic differences.
The UM remains the most plausible interpretation for these classes of objects
even if some results can be challenging for this model.Comment: 24 pages, 10 figures, conference proceeding "14th Serbian Conference
on Spectral Line Shapes in Astrophysics
Compact steep-spectrum sources as the parent population of flat-spectrum radio-loud NLS1s
Narrow-line Seyfert 1 galaxies (NLS1s) are an interesting subclass of active
galactic nuclei (AGN), which tipically does not exhibit any strong radio
emission. Seven percent of them, though, are radio-loud and often show a flat
radio-spectrum (F-NLS1s). This, along to the detection of -ray emission
coming from them, is usually interpreted as a sign of a relativistic beamed jet
oriented along the line of sight. An important aspect of these AGN that must be
understood is the nature of their parent population, in other words how do they
appear when observed under different angles. In the recent literature it has
been proposed that a specific class of radio-galaxies, compact-steep sources
(CSS) classified as high excitation radio galaxies (HERG), can represent the
parent population of F-NLS1s. To test this hypothesis in a quantitative way,in
this paper we analyzed the only two statistically complete samples of CSS/HERGs
and F-NLS1s available in the literature. We derived the black hole mass and
Eddington ratio distributions, and we built for the first time the radio
luminosity function of F-NLS1s. Finally, we applied a relativistic beaming
model to the luminosity function of CSS/HERGs, and compared the result with the
observed function of F-NLS1s. We found that compact steep-spectrum sources are
valid parent candidates and that F-NLS1s, when observed with a different
inclination, might actually appear as CSS/HERGs.Comment: 9 pages, 5 figures, 4 tables. Accepted for publication in Astronomy &
Astrophysic
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