Blazars, a class of active galactic nuclei emit over the entire accessible electromagnetic spectrum and modelling of their broadband 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 multi-wavelength data spanning over the period 2008 - 2018. Broadband SED modelling using γ-ray data from Fermi-Large Area Telescope, X-ray data from Swift-XRT, AstroSat, 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.
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 year Fermi γ-ray lightcurve 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 timescale 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. Broadband 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 γ-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 γ-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 γ-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.
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