959 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
Signatures of restarted activity in core-dominated, triple radio sources selected from the FIRST survey
Signatures of the re-occurrence of activity in radio-loud AGNs, indicated
either by the so-called double-double or X-shaped structures, have been
observed in a number of radio sources. All such objects known to date have
linear sizes of the order of a megaparsec. A number of the sources that are
appreciably more compact than this, but that exhibit hints of a past phase of
activity, were found in the VLA FIRST survey. Their structures show symmetric
relic lobes straddling relatively bright, unresolved cores. Observations of the
cores of 15 such structures with MERLIN at 5 GHz have shown that four of them
are doubles or core-jets on the subarcsecond scale. Misalignments of \Delta PA
\ga 30 degr. between the axis of the inner structure and the line connecting
the fitted maxima of the arcminute-scale relic lobes are clearly visible in
three of the four sources. From these results, we can infer that a rapid
repositioning of the central engine in each of these three radio sources is the
most plausible interpretation of the observed morphology and that a merger is
most likely the original cause of such a repositioning. In the case of TXS
1033+026, the optical image extracted from the SDSS archives clearly suggests
that two objects separated by only 2.7 kpc (projected onto the sky plane) are
indeed merging. The inner parts of TXS 0818+214 and TXS 1312+563 could be
interpreted as double-lobed, and consequently, these sources could be of the
double-double type; but further multifrequency observations are necessary to
provide support for such an interpretation.Comment: 9 pages, 5 figures, matches the version printed in Astronomy &
Astrophysics, very minor correction of Table
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