| dc.description.abstract | Metre-wavelength flux-density variations in compact extragalactic radio sources on time-scales of several months to a few years indicate brightness temperatures up to 3½ orders of magnitude higher than the inverse Compton limit of 10¹² K. In order to explain this in the light of the relativistically-expanding cloud model of Rees, very large values for the Doppler factor, T have to be invoked, implying an emitter moving relativistically at a small angle (1/T) to the line-of-sight. Statistically, one would then expect only a small fraction of sources to be suitably oriented to exhibit LFV. However, flux-density variations are rather common among compact extragalactic sources at both metre and centimetre wavelengths, leaving a small range of frequencies around 1 GHz where the variability seems to be a minimum, if not non-existent. This so-called ‘mid-frequency gap’ is not explained by the expanding-cloud model.
Alternative models invoking coherent emission or the light-echo phenomenon have been suggested, but none are entirely satisfactory.
The first attempt at an extrinsic explanation of LFV was the suggestion that the observed flux-density variations are due to varying absorption in a medium with changing opacity at some point along the line-of-sight. Later, Rickett, Coles and Bourgois pointed out that the observed LFV could be due to refractive interstellar scintillations (RISS). Many hitherto unexplained observations, such as the slow intensity fluctuations of pulsars, drifting bands in the dynamic spectra of pulsars and the “flickering” of extragalactic objects at centimetric wavelength, can also be explained by RISS.
Finding observational effects which support either the intrinsic or extrinsic scenarios has been the motivation for this thesis. Using the Ooty Synthesis Radio Telescope (OSRT), we have developed a method of flux-density estimation for sources of flux density > 1 Jy, giving an accuracy of 1% for an observing time of ~30 min. The method has been applied to a quarterly monitoring programme of a sample of 36 flat-spectrum sources over a period of 3 years. This sample was chosen to look for correlations between LFV and other intrinsic properties of the sources, such as the optical polarization. A comparative study of this dependence has been made at nine radio frequencies between 0.3 and 90 GHz, supplementing our measurements with published flux-density monitoring data.
We have investigated the association of the degree of radio intensity variations with the optical classification of the sources as low-polarization quasars (LPQs), high-polarization quasars (HPQs) and BL Lac objects. It is found that:
Below 1 GHz, the degrees of “short-term” variability (t ? 1 yr) exhibited by the three optical types are indistinguishable, pointing to an extrinsic origin for such variability. In contrast, longer-term variability may correlate with the optical classification.
At high frequencies (2.7–90 GHz), there is a distinct increase in the variability along the sequence LPQ–HPQ–BL Lac, supporting an intrinsic origin for HFV. Such a pattern of HFV persists even when only sources with ultra-flat radio spectra (? ? +0.2) are considered. Thus the radio compactness (reflected in ?) and the optical polarimetric/spectroscopic properties of AGNs appear to be independent factors associated with the nuclear activity. Further, the systematically different levels of HFV found for BL Lacs and HPQs place constraints on the models which seek to unify these optical types by postulating a selective amplification of the nonthermal optical continuum by effects such as gravitational micro-lensing.
A second sample of 96 compact sources was chosen to look for any dependence of variability parameters on galactic coordinates. This is expected if RISS is the major cause of LFV. Over a time interval of 15 years, we find that:
The modulation index is comparatively low for sources situated at low galactic latitudes (|b| < 5°).
There is a sudden rise in variability to a peak at |b| ? 15°.
At higher latitudes, the modulation index falls off and reaches almost a constant value for |b| > 45°.
When divided into two subgroups covering longitude ranges of 270° < l < 90° and 90° < l < 270°, the overall characteristics of the observed latitude dependencies are similar. We have tried to model this behaviour using different functional forms for the scattering strength, C?²(r,?,?), and have shown that a spiral model fits our data best.
The low-latitude sources from this sample for which structural information with mas resolution were not available at the time of forming the sample were observed using the European VLBI Network (EVN) at ? 6 and 18 cm. The results of these observations have established the presence of milliarcsecond structure in all of them, thereby eliminating the possibility that the low modulation index found from our data at low galactic latitudes is due to lack of compact structure in the sources.
Between these two wavelengths, we have detected angular broadening due to (diffractive) interstellar scattering for one source at l ? 4°, b = 0.47°. However, the amount of broadening is surprisingly small considering the proximity to the Galactic centre, where the compact nonthermal source, Sgr A, appears to be heavily scatter-broadened at low frequencies. Using the measurements of angular scattering for another extragalactic source at l = 0.82°, b = 0.18° (Anantharamaiah, private communication) and an OH-maser source in Sgr B2, we have tried to model the level and distribution of turbulence in the central 500 pc of the Galaxy. The picture that emerges from this study suggests that there is a very narrow central component of radial extent ~60 pc and a scale height of ~25 pc with an extremely high level of turbulence, C?² = 6.5 × 10?³ m??.??.
The observational effects presented in this thesis are consistent with the conclusion that most of LFV is an extrinsic effect caused by refractive interstellar scintillations. However, a few sources may undergo substantial intrinsic variations even at low frequencies. Superluminal sources with high measured values of the Doppler factor and slow variables at high galactic latitudes may be examples of such objects. On the whole, it appears reasonable to decouple LFV from the physics of compact extragalactic radio sources. Instead, as we have demonstrated here, the phenomenon may be used to understand the nature and distribution of turbulence in the ionized component of the interstellar medium. | |
