Blazars, a class of active galactic nuclei, emit over the entire accessible electromagnetic spectrum and modelling of their broad-band spectral energy distribution (SED) is the key to constrain the underlying emission mechanisms. Here we report the results on the one-zone leptonic emission modelling carried out on the blazar 4C+21.35 using multiwavelength data spanning over the period 2008–2018. Broad-band SED modelling using γ-ray data from Fermi-Large Area Telescope, X-ray data from Swift-XRT and AstroSat, and UV–optical data from Swift-UVOT, AstroSat, and Catalina Real-Time Transient Survey was carried out at seven different epochs, including three γ-ray flaring episodes and four quiescent periods (three long-term averaged ones and one during AstroSat observing period). Our SED modelling suggests that two compact emission regions originating at a different time outside the broad-line region and moving away from the core with variation primarily in the jet electron spectra can explain the emission from the high-, moderate-, and low-activity periods. The emissions from high- and first low-activity states are likely to have originated in the first region. The moderate- and second low-activity states are likely due to the second emission region with fresh particle acceleration/injection at a later time.

We present the results of our study of cross-correlations between long-term multi-band observations of the radio variability of the blazar 3C 279. More than a decade (2008−2022) of radio data were collected at seven different frequencies ranging from 2 GHz to 230 GHz. The multi-band radio light curves show variations in flux, with the prominent flare features appearing first at higher-frequency and later in lower-frequency bands. This behavior is quantified by cross-correlation analysis, which finds that the emission at lower-frequency bands lags that at higher-frequency bands. Lag versus frequency plots are well fit by straight lines with negative slope, typically ∼−30 day GHz⁻¹. We discuss these flux variations in conjunction with the evolution of bright moving knots seen in multi-epoch VLBA maps to suggest possible physical changes in the jet that can explain the observational results. Some of the variations are consistent with the predictions of shock models, while others are better explained by a changing Doppler beaming factor as the knot trajectory bends slightly, given a small viewing angle to the jet.

We report the results of our long-term multiwavelength spectral energy distribution (SED) study on the flat-spectrum radio quasar 3C 279 during the ∼14 yr (2008–2022) Fermi-Large Area Telescope (LAT) observing period. The Fermi-LAT data were complemented with data in other wave bands obtained from the Swift-X-Ray Telescope (XRT)/UVOT, the Whole Earth Blazar Telescope, along with other optical and radio data from several observatories. Different activity states were identified from the weekly binned γ-ray light curve, and it was possible to create 168 high-quality and quasi-simultaneous broadband SEDs. We modeled the SEDs using a one-zone leptonic scenario, including the emission region outside the broad-line region (BLR), involving synchrotron, synchrotron self-Compton, and external Compton mechanisms. Such extensive broadband modeling is essential for constraining the underlying multiwavelength radiative mechanisms in the 3C 279 jet and permits the estimation of the physical parameters and exploration of their evolution in time. Our SED modeling study suggests that the increase in the Doppler beaming factor, along with the variation of the emitting electrons, is the cause for the flares in this source. The multiwavelength emission of 3C 279 was found to be well explained by the scenario in which the emission region is outside the BLR at a distance of ∼6.42 × 10³ R _S . However, for two of the very bright γ-ray states, the emission region was found to be close to the outer boundary of the BLR at a distance of ∼1.28 × 10³ R _S from the central black hole.

We examine the long-term (∼10 years) γ-ray variability of blazars observed by the Energetic Gamma Ray Experiment Telescope (EGRET) and Fermi and find that 10 EGRET-detected flat-spectrum radio quasars (FSRQs) were not present in any part of the Fermi catalogue (during the first four years of Fermi observations). Six of these 10 EGRET sources show possible Fermi counterparts, based on overlap with error circles given in the third Fermi catalogue. A seventh EGRET source is positionally consistent with a newly discovered Fermi source from our analysis. Unlike the high luminosity state exhibited during 1991–1995, our analysis using seven years of Fermi data showed that five of these sources are in a low luminosity state in the GeV band. This result indicates that FSRQs exhibit a long-term quiescent state (lasting more than seven years). In addition, 37 new γ-ray sources are also detected during the course of this analysis. On the basis of active galactic nuclei (AGNs) being the most dominant source class in γ-rays, many of these new sources could be associated with AGNs and may exhibit long-term variabilities in γ-rays.

Blazar variability can be described as flaring activities on a wide range of time-scales over a baseline flux level. It is important to detect and distinguish baseline flux changes from long-term flare variations, since the former may reflect state transitions caused by a secular change in bulk properties such as the bulk Lorentz factor or the viewing angle. We report such a transition observed in the 11 yr Fermi γ-ray light curve of the blazar 3C 66A, where the baseline flux of ∼1.8 × 10⁻⁷ ph cm⁻² s⁻¹ persisted for three years and then changed over a month time-scale to ∼0.8 × 10⁻⁷ ph cm⁻² s⁻¹ and remained in that level for the next eight years. Moreover, there is evidence for a similar shift in baseline flux in the optical band. Broad-band spectral energy distribution modelling for the two activity states reveals that the baseline flux change is consistent with an overall decrease in the Doppler factor, which in turn implies an increase in the jet inclination angle by ∼1° or a decrease in the bulk Lorentz factor by ∼25 per cent. We discuss the implication of such a variation occurring on a month time-scale.

The blazar PKS 0208-512 was in the lowest $\gamma$-ray brightness state during the initial 10 years of observations with the Fermi Gamma-ray Space Telescope (Fermi), which was an order of magnitude lower than its flux state during the EGRET era (1991–2000). The weekly averaged maximum $\gamma$-ray flux of this source during the first 10 years of Fermi observation is nearly a factor of 3 lower than the highest flux observed by EGRET in a single epoch. During the period 2018–2020, the source showed a large $\gamma$-ray flare, with the average brightness similar to the period 1991–2000. We observed the source with AstroSat, during its low and high activity states, respectively. We carried out broad-band spectral energy distribution (SED) modeling of the source using a one-zone leptonic emission model during its various brightness states. From the SED modeling, we found that there was an inefficient conversion from the bulk energy to the particle energy during the long-term low-activity states as compared to the high flux state during the EGRET era and the later part of Fermi observation.