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Register a new userTesting adiabatic expansion of shocks in parsec-scale jets by dual-frequency VLBI experiments
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A. B. Pushkarev
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We present results of simultaneous dual-frequency (2 GHz and 8 GHz) very long baseline interferometry (VLBI) observations of 12 active galactic nuclei with prominent jets. Spectral properties of the jets and evolution of their brightness temperature are discussed. Measured sizes and brightness temperatures of VLBI features are found to be consistent with emission from relativistic shocks dominated by adiabatic losses. Physical scenarios with different magnetic field orientation in the jets are discussed.
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Multi-frequency VLBI observations allow studies of the continuum spectrum in the different parts of the parsec scale jets of AGN, providing information on the physical properties of the plasma and magnetic fields in them. Since VLBI networks cannot be scaled, the range of spatial frequencies observed differs significantly between the different observing frequencies, which makes it difficult to obtain a broadband spectrum of the individual emission features in the jet. In this paper we discuss a model-fitting based spectral extraction method, which can significantly relieve this problem. The method uses a priori knowledge of the source structure, measured at high frequencies, to allow at lower frequencies the derivation of the sizes and flux densities of even those emission features that have mutual separations significantly less than the Rayleigh limit at the given frequency. We have successfully used this method in the analysis of 5-86 GHz VLBA data of 3C273. The spectra and sizes of several individual jet features were measured, thus allowing derivation of the magnetic flux density and the energy density of the relativistic electrons in the different parts of the jet. We discuss the results, which include e.g. a detection of a strong gradient in the magnetic field across the jet of 3C273.
We report on an ongoing effort to image active galactic nuclei simultaneously observed at 2.3 and 8.6 GHz in the framework of a long-term VLBI project RDV (Research and Development - VLBA) started in 1994 aiming to observe compact extragalactic radio sources in the astrometric/geodetic mode. Observations of bright extragalactic sources are carried out bi-monthly making up to six sessions per year with participation of all ten VLBA antennas and up to nine additional (geodetic and EVN) radio telescopes. Analysis of single-epoch results for 370 quasars, BL Lacs and radio galaxies is presented. We discuss VLBI core properties (flux densities, sizes, brightness temperatures), spectral characteristics of the cores and jets, evolution of brightness temperatures in the jets.
We conducted radio detection observations at 8.4 GHz for 22 radio-loud broad absorption line (BAL) quasars, selected from the Sloan Digital Sky Survey (SDSS) Third Data Release, by a very-long-baseline interferometry (VLBI) technique. The VLBI instrument we used was developed by the Optically ConnecTed Array for VLBI Exploration project (OCTAVE), which is operated as a subarray of the Japanese VLBI Network (JVN). We aimed at selecting BAL quasars with nonthermal jets suitable for measuring their orientation angles and ages by subsequent detailed VLBI imaging studies to evaluate two controversial issues of whether BAL quasars are viewed nearly edge-on, and of whether BAL quasars are in a short-lived evolutionary phase of quasar population. We detected 20 out of 22 sources using the OCTAVE baselines, implying brightness temperatures greater than 10^5 K, which presumably come from nonthermal jets. Hence, BAL outflows and nonthermal jets can be generated simultaneously in these central engines. We also found four inverted-spectrum sources, which are interpreted as Doppler-beamed, pole-on-viewed relativistic jet sources or young radio sources: single edge-on geometry cannot describe all BAL quasars. We discuss the implications of the OCTAVE observations for investigations for the orientation and evolutionary stage of BAL quasars.
The upper envelope of the amplitude of the VLBI visibility function usually represents the most compact structural pattern of extragalactic radio sources, in particular, the core-jet morphologies. By fitting the envelope to a circular Gaussian model in ~3000 parsec-scale core-jet structures, we find that the apparent angular size shows significant power-law dependence on the observing frequency (power index n = -0.95 pm 0.37). The dependence is likely to result from synchrotron self-absorption in the inhomogeneous jet and not the free-free absorption (n = -2.5), nor the simple scatter broadening (n leq -2).
We report the discovery of multi-scale X-ray jets from the accreting neutron star X-ray binary, Circinus X-1. The bipolar outflows show wide opening angles and are spatially coincident with the radio jets seen in new high-resolution radio images of the region. The morphology of the emission regions suggests that the jets from Circinus X-1 are running into a terminal shock with the interstellar medium, as is seen in powerful radio galaxies. This and other observations indicate that the jets have a wide opening angle, suggesting that the jets are either not very well collimated or precessing. We interpret the spectra from the shocks as cooled synchrotron emission and derive a cooling age of approximately 1600 yr. This allows us to constrain the jet power to be between 3e35 erg/s and 2e37 erg/s, making this one of a few microquasars with a direct measurement of its jet power and the only known microquasar that exhibits stationary large-scale X-ray emission.
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