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تسجيل مستخدم جديدNew VLBA Identifications of Compact Symmetric Objects
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The class of radio sources known as Compact Symmetric Objects (CSOs) is of particular interest in the study of the evolution of radio galaxies. CSOs are thought to be young (probably ~10^4 years), and a very high fraction of them exhibit HI absorption toward the central parsecs. The HI, which is thought to be part of a circumnuclear torus of accreting gas, can be observed using the VLBA with high enough angular resolution to map the velocity field of the gas. This velocity field provides new information on the accretion process in the central engines of these young sources. We have identified 9 new CSOs from radio continuum observations for the VLBA Calibrator Survey, increasing the number of known CSOs by almost 50%.
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We present results of multifrequency polarimetric VLBA observations of 34 compact radio sources. The observations are part of a large survey undertaken to identify CSOs Observed in the Northern Sky (COINS). Compact Symmetric Objects (CSOs) are of par
ticular interest in the study of the physics and evolution of active galaxies. Based on VLBI continuum surveys of ~2000 compact radio sources, we have defined a sample of 52 CSOs and CSO candidates. In this paper, we identify 18 previously known CSOs, and introduce 33 new CSO candidates. We present continuum images at several frequencies and, where possible, images of the polarized flux density and spectral index distributions for the 33 new candidates and one previously known but unconfirmed source. We find evidence to support the inclusion of 10 of these condidates into the class of CSOs. Thirteen candidates, including the previously unconfirmed source, have been ruled out. Eleven sources require further investigation. The addition of the 10 new confirmed CSOs increases the size of this class of objects by 50%.
B2352+495 is a prototypical example of a Compact Symmetric Object (CSO). It has a double radio lobe symmetrically located with respect to a central flat spectrum radio core (the location of the AGN) and has a physical extent of less than 200 pc. In t
his work we report VLBA observation of 21 cm HI absorption toward B2352+495 to investigate the properties of this remarkable radio source, in particular, to explore whether the radio emission can be confined by circumnuclear material (frustration scenario) or whether the source is likely to be young. We confirmed the two HI absorption features previously detected toward B2352+495 - a broad line nearly centered at the systemic velocity of the galaxy and a narrow redshifted component. The atomic gas from the broad absorption component is likely associated with circumnuclear material, consistent with the current paradigm of clumpy HI distribution in toroidal structures around supermassive black holes.
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 bl
ack 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.
We present results from the first intensive monitoring campaign of a sample of Compact Symmetric Objects (CSOs). We observed seven CSOs at 8.5 GHz over a period of eight months, with an average spacing between observations of 2.7 days. Our results sh
ow that, as predicted, the flux densities of the CSOs are extremely stable; the mean RMS variability of the sample was 0.7% in flux density. The low variability of the CSOs makes them excellent flux density calibrators at this frequency. We recommend that at least four CSOs be included in any VLA monitoring campaign which requires precise epoch-to-epoch calibration, such as those to measure gravitational lens time delays. The CSO data enable the correction of small systematic errors in the primary flux calibration.
We report results on multi-epoch Very Large Array (VLA) and pc-scale Very Long Baseline Array (VLBA) observations of candidate compact symmetric objects (CSOs) from the faint sample of high frequency peakers. New VLBA observations could resolve the r
adio structure in about 42 per cent of the observed sources, showing double components that may be either mini-lobes or core-jet structures. Almost all the sources monitored by the VLA show some variability on time scale of a decade, and only 1 source does not show any significant variation. In 17 sources the flux density changes randomly as it is expected in blazars, and in 4 sources the spectrum becomes flat in the last observing epoch, confirming that samples selected in the GHz regime are highly contaminated by beamed objects. In 16 objects, the pc-scale and variability properties are consistent with a young radio source in adiabatic expansion, with a steady decrease of the flux density in the optically-thin part of the spectrum, and a flux density increase in the optically-thick part. For these sources we estimate dynamical ages between a few tens to a few hundreds years. The corresponding expansion velocity is generally between 0.1c and 0.7c, similar to values found in CSOs with different approaches. The fast evolution that we observe in some CSO candidates suggests that not all the objects would become classical Fanaroff-Riley radio sources.
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