26 research outputs found
Orientation of the cores of hybrid morphology radio sources
The FRI/FRII dichotomy is a much debated issue in the astrophysics of
extragalactic radio sources. Study of the properties of HYbrid MOrphology Radio
Sources (HYMORS) may bring crucial information and lead to a step forward in
understanding the origin of FRI/FRII dichotomy. HYMORS are a rare class of
double-lobed radio sources where each of the two lobes clearly exhibits a
different FR morphology. This article describes follow-up high resolution VLBA
observations of the five discovered by us HYMORS. The main aim of the
observations was to answer the questions of whether the unusual radio
morphology is connected to the orientation of objects towards the observer. We
obtained the high resolution radio maps of five hybrid radio morphology objects
with the VLBA at C-band and L-band. Two of them revealed milliarcsecond
core-jet structures, the next two objects showed hints of parsec-scale jets,
and the last one remained point-like at both frequencies. We compared
properties of observed milliarcsecond structures of hybrid sources with the
larger scale ones previously detected with the VLA. We find that on both scales
the fluxes of their central components are similar, which may indicate the lack
of additional emission in the proximity of the nucleus. This suggests that jets
present on the 1-10 kpc scale in those objects are FRII-like. When possible,
the detected core-jet structures were used for estimating the core's spatial
orientation. The result is that neither the FRI-like nor the FRII-like side is
preferred, which may suggest that no specific spatial orientation of HYMORS is
required to explain their radio morphology. Their estimated viewing angles
indicate they are unbeamed objects. The 178 MHz luminosity of observed HYMORS
exceed the traditional FRI/FRII break luminosity, indicating they have radio
powers similar to FRIIs.Comment: 7 pages, 5 figures, accepted for publication in A&
FIRST-based survey of Compact Steep Spectrum sources, IV. Multifrequency VLBA observations of very compact objects
Evidence has been mounting recently that activity in some radio-loud AGNs
(RLAGNs) can cease shortly after ignition and that perhaps even a majority of
very compact sources may be short-lived phenomena because of a lack of stable
fuelling from the black hole. Thus, they can fade out before having evolved to
large, extended objects. Re-ignition of the activity in such objects is not
ruled out. With the aim of finding more examples of these objects and to
investigate if they could be RLAGNs switched off at very early stages of their
evolution, multifrequency VLBA observations of six sources with angular sizes
significantly less than an arcsecond, yet having steep spectra, have been made.
Observations were initially made at 1.65 GHz using the VLBA with the inclusion
of Effelsberg telescope. The sources were then re-observed with the VLBA at 5,
8.4 and 15.4 GHz. All the observations were carried out in a snapshot mode with
phase referencing. One of the sources studied, 0809+404, is dominated by a
compact component but also has diffuse, arcsecond-scale emission visible in VLA
images. The VLBI observations of the "core" structure have revealed that this
is also diffuse and fading away at higher frequencies. Thus, the inner
component of 0809+404 could be a compact fading object. The remaining five
sources presented here show either core-jet or edge-brightened double-lobed
structures indicating that they are in an active phase. The above result is an
indication that the activity of the host galaxy of 0809+404 may be
intermittent. Previous observations obtained from the literature and those
presented here indicate that activity had ceased once in the past, then
restarted, and has recently switched off again.Comment: 15 pages, 7 figures, matches the version printed in Astronomy &
Astrophysic
Stability of black hole accretion disks
We discuss the issues of stability of accretion disks that may undergo the
limit-cycle oscillations due to the two main types of thermal-viscous
instabilities. These are induced either by the domination of radiation pressure
in the innermost regions close to the central black hole, or by the partial
ionization of hydrogen in the zone of appropriate temperatures. These physical
processes may lead to the intermittent activity in AGN on timescales between
hundreds and millions of years. We list a number of observational facts that
support the idea of the cyclic activity in high accretion rate sources. We
conclude however that the observed features of quasars may provide only
indirect signatures of the underlying instabilities. Also, the support from the
sources with stellar mass black holes, whose variability timescales are
observationally feasible, is limited to a few cases of the microquasars.
Therefore we consider a number of plausible mechanisms of stabilization of the
limit cycle oscillations in high accretion rate accretion disks. The newly
found is the stabilizing effect of the stochastic viscosity fluctuations.Comment: 4 pages, 4 figures; proceedings of the "AGN Outbursts and Accretion
Physics Workshop", 25-27 June 2012, Madri
Changing-look AGNs or short-lived radio sources?
The evolution of extragalactic radio sources has been a fundamental problem
in the study of active galactic nuclei for many years. A standard evolutionary
model has been created based on observations of a wide range of radio sources.
In the general scenario of the evolution, the younger and smaller
Gigahertz-Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources become
large-scale FRI and FRII objects. However, a growing number of observations of
low power radio sources suggests that the model cannot explain all their
properties and there are still some aspects of the evolutionary path that
remain unclear. There are indications, that some sources may be short-lived
objects on timescales of - years. Those objects represent a new
population of active galaxies. Here, we present the discovery of several radio
transient sources on timescales of 5-20 years, largely associated with renewed
AGN (Active Galactic Nucleus) activity. These changing-look AGNs possibly
represent behaviour typical for many active galaxies.Comment: 5 pages, 3 figures, contribution to proceedings of the conference
"Quasars at all cosmic epochs
Caltech-NRAO Stripe 82 Survey (CNSS). IV. The Birth of Radio-loud Quasar 013815+00
It is believed that the gas accretion onto supermassive black holes is the main process of powering this quasar's luminous emission, which occurs in optical, UV, and X-ray regimes and less frequently in radio waves. The observational fact that only a few percent of quasars are radio-loud is still an unresolved issue concerning the understanding of the active galactic nucleus (AGN) population. Here we present a detection of a rapid transition from the radio-quiet to the radio-loud mode in quasar 013815+00 (z = 0.94) which coincides with changes of its UV–optical continuum and the low ionization Mg II broadline. We interpret this as an enhancement of accretion onto a central black hole of about 10⁹ solar masses. As a consequence a new radio-loud AGN was born. Its spectral and morphological properties indicate that it went through the short gigahertz-peaked spectrum phase at the beginning of its activity and has now stabilized its flux density at the level of a few millijansky. The radio morphology of 013815+00 is very compact and we predict that with such short-term jet activity its development will be very slow. The observed luminosity changes of the accretion disk are shorter than the lifetime of the new radio phase in 013815+00