We present a statistical characterization of the γ-ray emission from the four Fermi-LAT sources: FR I radio galaxy NGC 1275, BL Lac Mrk 421, FSRQs B2 1520+31, and PKS 1510-089 detected almost continuously over a time integration of three days between 2008 August and 2015 October. The observed flux variation is large, spanning orders of magnitude between the extremes, except for Mrk 421. We compute the flux distributions and compare with Gaussian and lognormal ones. We find that the three blazars have distributions consistent with a lognormal, suggesting that the variability is of a nonlinear, multiplicative nature. This is further supported by the computation of the flux-rms relation, which is observed to be linear for the three blazars. However, for NGC 1275, the distribution does not seem to be represented either by a lognormal or a Gaussian, while its flux-rms relation is still found to be linear. We also compute the power spectra, which suggest the presence of a break, but are consistent with typical scale-free power-law shot noise. The results are broadly consistent with the statistical properties of the magnetic reconnection powered minijets-in-a-jet model. We discuss other possible scenarios and implications of these observations on jet processes and connections with the central engine.

We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 – May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at γ-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR–UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible–UV bands, never seen before in this source. The optical bump can be explained as accretion-disk emission associated with the primary black hole of mass $$\sim \rm 1.8 \times 10^{10} M_{\odot }$$ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broadband SEDs extracted during the first flare and during a quiescent period during this span show very different γ-ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since May 2013, as found by examining all available NIR-optical observations, and favors the binary super-massive black hole model. The simultaneous multi-wavelength variability and relatively weak γ-ray emission that shows a shift in the SED peak is consistent with γ-ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump.

We present a systematic characterization of multi-wavelength emission from blazar PKS 1510-089 using well-sampled data at near-infrared (NIR), optical, X-ray, and γ-ray energies. The resulting flux distributions, except at X-rays, show two distinct lognormal profiles corresponding to a high and a low flux level. The dispersions exhibit energy-dependent behavior except in the LAT γ-ray and optical B-band. During the low level flux states, it is higher toward the peak of the spectral energy distribution, with γ-ray being intrinsically more variable followed by IR and then optical, consistent with mainly being a result of varying bulk Lorentz factor. On the other hand, the dispersions during the high state are similar in all bands except the optical B-band, where thermal emission still dominates. The centers of distributions are a factor of ∼4 apart, consistent with anticipation from studies of extragalactic γ-ray background with the high state showing a relatively harder mean spectral index compared to the low state.

We present recent optical photometric observations of the blazar OJ 287 taken during 2015 September–2016 May. Our intense observations of the blazar started in 2015 November and continued until 2016 May and included detection of the large optical outburst in 2015 December that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of nine ground-based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ∼1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in 2016 March. In addition, we investigated multiband flux variations, colour variations, and spectral changes in the blazar on diverse time-scales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour, and spectral variability.

We detected a possible quasi-periodic oscillation (QPO) of ∼ 71 days in the 0.1 – 300 GeV γ-ray Fermi-LAT light curve of the high redshift flat spectrum radio quasar B2 1520+31. We identify and confirm that quasi-period by Lomb Scargle periodogram (LSP) and weighted wavelet z-transform (WWZ) analyses. Using this QPO period, and assuming it originates from accretion-disc fluctuations at the innermost stable circular orbit, we estimate the central supermassive black hole mass to range between ∼5.4 × 10⁹ M_⊙ for a non-rotating black hole and ∼3.4 × 10¹⁰ M_⊙ for a maximally rotating black hole. We briefly discuss other possible radio-loud active galactic nuclei emission models capable of producing a γ-ray QPO of such a period in a blazar.

The blazar 3C 454.3 exhibited a strong flare seen in γ-rays, X-rays and optical/near-infrared bands during 2009 December 3–12. Emission in the V and J bands rose more gradually than did the γ-rays and soft X-rays, though all peaked at nearly the same time. Optical polarization measurements showed dramatic changes during the flare, with a strong anticorrelation between optical flux and degree of polarization (which rose from ∼3 to ∼20 per cent) during the declining phase of the flare. The flare was accompanied by large rapid swings in polarization angle of ∼170°. This combination of behaviours appears to be unique. We have cm-band radio data during the same period but they show no correlation with variations at higher frequencies. Such peculiar behaviour may be explained using jet models incorporating fully relativistic effects with a dominant source region moving along a helical path or by a shock-in-jet model incorporating three-dimensional radiation transfer if there is a dominant helical magnetic field. We find that spectral energy distributions at different times during the flare can be fit using modified one-zone models where only the magnetic field strength and particle break frequencies and normalizations need change. An optical spectrum taken at nearly the same time provides an estimate for the central black hole mass of ∼2.3 × 10⁹ M_⊙. We also consider two weaker flares seen during the ∼200 d span over which multiband data are available. In one of them, the V and J bands appear to lead the γ-ray and X-ray bands by a few days; in the other, all variations are simultaneous.