Multifrequency connection in blazars: Tool for studying the location and emission mechanisms

Blazars are a sub-class of AGN with relativistic jets oriented towards the line of sight of the observer. The radiation mechanism in the radio, and sometimes in optical and X-rays, are dominated by synchrotron processes with high-energy emission arising from the inverse-Compton scattering of photons by the relativistic electrons or through hadronic processes. The latter scenario is one of the topics debated in the blazar research, primarily owing to the location of the seed photons for the high-energy emission. This problem is contemplated in this thesis for a sample of blazars. A proper understanding of the physical mechanisms responsible for variability is contingent upon a mathematical and statistical description of the phenomena. Hence, the variability of all light curves was investigated using power spectral density and Bayesian blocks, and its connection using the discrete correlation function. The connection between the variability at 37 GHz and gamma-ray light curves was studied in 55 blazars. The average time delay from this study showed that the variations in the radio lagged behind those at gamma rays. However, by taking the size of the radio core into account the gamma-ray emission region was localised to the radio core in the majority of sources. In another work, the cross-correlations were performed for 15 blazars using light curves at 37 GHz, 95 GHz, R-band and gamma rays. Due to the shorter time span of the light curves and undersampled optical data in some cases, significant correlations were obtained only in a few sources. The variations at 37 GHz and 95 GHz were simultaneous in most of the sources, indicating that the opacity constraints at both these frequencies were similar. The connection between the events in the radio through gamma rays was studied in the blazars 1156+295, Mrk 421 and OJ 248. From the correspondence of the activity at both radio and gamma rays in 1156+295, and also based on the ejection time of the shocks from the Very Long Baseline Array analysis, a causal connection constraining the gamma-ray flares to the parsec-scale radio jet is established. The physical explanation for the radio variability in Mrk 421 is discussed by modelling the radio and gamma-ray light curve under a one-zone SSC framework. For OJ 248, a multifrequency connection for the 2012 gamma-ray flare is explored using cross-correlations. To understand the synchrotron processes, the radio spectra of 104 AGN from Planck satellite observations are analysed. The shape of most of the spectra is consistent with those predicted by the shock-in-jet model. The overall conclusion from this thesis is that in most of the blazars studied the gamma-ray emission arises from the parsec-scale radio jet, which supports the SSC mechanism (through one- or multi-zone models) for the high-energy emission.