1,018 research outputs found
Intermittent activity of radio sources. Accretion instabilities and jet precession
We consider the radiation pressure instability operating on short timescales
10^3 - 10^6 years in the accretion disk around a supermassive black hole as the
origin of the intermittent activity of radio sources. We test whether this
instability can be responsible for short ages (<10^4 years) of Compact Steep
Spectrum sources measured by hot spots propagation velocities in VLBI
observations and statistical overabundance of Gigahertz Peaked Spectrum
sources.The implied timescales are consistent with the observed ages of the
sources. We aslo discuss possible implications of the intermittent activity on
the complex morphology of radio sources, such as the quasar 1045+352, dominated
by a knotty jet showing several bends. It is possible that we are whitnessing
an ongoing jet precession in this source due to internal instabilities within
the jet flow.Comment: 2 pages, 1 figure, Proceedings of the 275 IAU Symposium "Jets at all
scales", Buenos Aires, 13-17.09.2010; eds. G. Romero, R. Sunyaev, T. Bellon
A survey of Low Luminosity Compact sources and its implication for evolution of radio-loud AGNs. I. Radio data
We present a new sample of Compact Steep Spectrum (CSS) sources with radio
luminosity below 10^26 W/Hz at 1.4 GHz called the low luminosity compact (LLC)
objects. The sources have been selected from FIRST survey and observed with
MERLIN at L-band and C-band. The main criterion used for selection was
luminosity of the objects and approximately one third of the CSS sources from
the new sample have a value of radio luminosity comparable to FRIs. About 80%
of the sources have been resolved and about 30% of them have weak extended
emission and disturbed structures when compared with the observations of higher
luminosity CSS sources. We studied correlation between radio power and linear
size, and redshift with a larger sample that included also published samples of
compact objects and large scale FRIIs and FRIs. The low luminosity compact
objects occupy the space in radio power versus linear size diagram below the
main evolutionary path of radio objects. We suggest that many of them might be
short-lived objects, and their radio emission may be disrupted several times
before becoming FRIIs. We conclude that there exists a large population of
short-lived low luminosity compact objects unexplored so far and part of them
can be precursors of large scale FRIs.Comment: 19 pages, 5 figures, 3 tables, MNRAS in pres
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
- …