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Relativistic beaming in active galactic nuclei

Shastri, Prajval

Relativistic beaming in active galactic nuclei [Ph.D Thesis] Prajval Shastri - Bombay Tata Institute of Fundamental Research 1989 - viii, 180p.

Several of the observed properties of radio-powerful active galactic nuclei can be understood if Doppler enhancement of the radio continuum radiation due to bulk relativistic motion in their nuclear regions is invoked. In particular> in the framework of the "unified interpretation" for the radio emission from quasars (Blandford & K5nigl, 1979; Kapahi & Saikia, 1982; Orr & Browne, 1982), the apparent differences between objects with dominant radio lobes (steep overall spectrum at radio wavelengths) and those with dominant nuclear radio components (flat overall radio spectrum) is a consequence of the differing angles from which the objects are viewed. Because only the radiation from the nuclear region is assumed to be beamed, the ratio of the radio emission from the nuclear component to that from the extended structure is a statistical measure of the angle of orientation of the nuclear jet to the line of sight. In this thesis, some of the observed properties of radio quasars have been explored in order to ascertain their consistency with the relativistic beaming hypothesis. The data used include observations reported as a part of this thesis work (which are mainly at radio wavelengths) and also those gleaned from the literature. The results are generally consistent with the predictions of the "unified scheme". Firstly, the question of bulk relativistic velocities in the outer radio components has been investigated. While most radio-powerlul active galactic nuclei which show extended radio structure have lobes of radio emission on opposite sides of the nuclear component, a significant number have been known to show extensions on only one side. That the twin radio lobes are due to radiatively dissipative plasma that is squirted out on opposite sides of the active nucleus is now conventional wisdom. The special feature of "one-sidedness" of radio structure seen in some of them is sought to be explained in the framework of the unified interpretation, by attributing it to Doppler enhancement of the flux density from the approaching outer component relative to that from the receding one at small viewing angles. To examine this hypothesis in detail, a sample of about forty quasars showing "onesided" structure has been chosen and imaged at radio wavelengths (in both total intensity and linear polarization) with high angular resolution. The Very Large Array synthesis telescope of the National Radio Astronomy Observatory, U.S.A. has been used for the imaging. These observations have been used to confirm the radio structure of the objects and make a systematic study of their properties. The results, briefly, are as follows. Many of the objects have two-sided structure that is mostly very asymmetric (spatially and/or in surface brightness) with respect to the nuclear component. It appears that the earlier observations, on the basis of which the classification as "one-sided" was made, had missed the second of the twin lobes due to insufficient angular resolution, sensitivity or dynamic range. Radio jets have been detected in many cases, with structural and (inferred) magnetic field properties that are characteristic of quasars. The radio jet points towards the brighter of the two hot spots in almost every case, consistent with the supposition that at least mild bulk relativistic motion is present in the hot spots. A comparison with other quasar samples indicates that relatively large asymmetry in peak surface brightness of the outer hot spots is associated with relatively dominant nuclear components and smaller projected linear sizes, again consistent with the hypothesis. The unified interpretation is not valid for the class of Compact Steep Spectrum Quasars, where observations suggest that interaction of the radio jets with a possibly gas-rich interstellar medium may play an important role. Secondly, the question of misalignments of the nuclear jets has been explored using inferences from radio polarimetry. According to the unified scheme. coredominated quasars are inclined at small angles to the line of sight, and therefore are expected to show relatively large misalignments of the nuclear jet with respect to the overall radio axis. To test this prediction for a large sample of quasars, polarimetry of the nuclear radio component at 5 GHz was used to infer the direction of the nuclear jet. It is assumed that the magnetic field in the nuclear jets orients along the jet (which has been observed to be the case in several quasars). The test shows that while there are large misalignments in evidence in core-dominated quasars, the nuclear jets appear well-aligned with the overall radio axis in lobedominated quasars, which is consistent with the prediction. An analogous investigation for the case of radio galaxies also reveals a trend toward alignment. Thirdly, the possibility that the optical-ultraviolet nuclear emission may be aspect dependent in quasars has been examined. If the unified scheme is valid, it follows that relativistic beaming effects at radio wavelengths are accompanied by an apparent enhancement of the optical-ultraviolet continuum emission from the nucleus in a quasar (Browne & Wright, 1985). Several other observed properties of radio quasars are found here to be consistent with this interpretation. The prominence of the radio nuclear component is shown to directly correlate with the apparent optical brightness, and also with the ratio of nuclear to host galaxy emission at optical wavelengths. These observations can be understood if the nuclear optical continuum is aspect dependent in the same sense as the nuclear radio emission is. There is a marginal trend for radio jets of kiloparsec scales to occur more frequently among the optically brighter quasl;U's. This is to be expected if the optical continuum is aspect dependent and there is bulk relativistic motion at kiloparsec scales. The aspect dependence could arise from relativistic beaming of optical synchrotron emission, or, alternatively, from inclination effects of radiation from a hot optically thick but geometrically thin accretion disc. It appears that both these components might be significant contributors to the optical-ultraviolet continuum emission from these objects. The two would, however, show quite different aspectdependent behaviour, and therefore dominate to different degrees depending on the aspect. Some of the implications of the aspect dependence of the optical continuum have then been investigated. Optical aspect dependence can introduce a serious orientation bias into existing samples of quasars even when selected at low radio frequencies. (At these frequencies, they get selected by their extended radio emission that is not strongly Doppler beamed). The relativistically beamed component of the optical continuum would contribute to increasing the scatter in the Hubble diagram of fiat spectrum (core-dominated) quasars; on the other hand. the aspect dependence of the disc component would increase the scatter for the steep spectrum (lobe-dominated) quasars. Lastly, limited evidence has been found that the enhancement of the nuclear radio emission might be accompanied by enhancement of a component of the x-ray emission from these nuclei. In conclusion, the evidence m favour of the unified scheme is summarized. Some observational results in the literature that do not support the scheme are also discussed. Adopting the point of view that the unified scheme and its implications suggest and promise significant order and progress in the search for a unified phenomenology for active galactic nuclei, some directions in which observational investigations might be profitably pursued are pointed out.

Active galactic nuclei
Radio spectra
Relativistic beaming

(043.2):524.64 / SHA

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