We present a multi-wavelength study of the FSRQ CTA 102 using Fermi-LAT and simultaneous Swift-XRT/UVOT observations. The Fermi-LAT telescope detected one of the brightest flares from this object during Sep, 2016 to Mar, 2017 In the 190 days of observation period the source underwent four major flares. A detailed analysis of the temporal and spectral properties of these flares indicates the flare at MJD 57751.594 has a γ-ray flux of (30.12±4.48)×10 −6 ph cm −2 s −1 (from 90 minutes binning) in the energy range of 0.1-300 GeV. This has been found to be the highest flux ever detected from CTA 102. Time dependent leptonic modelling of the pre-flare, rising state, flares and decaying state has been done. A single emission region of size 6.5 × 10 16 cm has been used in our work to explain the multi-wavelength spectral energy distributions. During flares the luminosity in electrons increases nearly seventy times compared to the pre-flare state.

We consider the response of an overdamped nonlinear oscillator driven by additive Gaussian white noise to an additive weak harmonic forcing in the context of stochastic resonance (SR). We show that, in addition to the previously reported mechanisms, there exists another type of SR, which can occur even when the system is monostable. If the system is multistable, SR of this new type can coexist with the conventional SR in an appropriate frequency range. Further immediate extensions include stochastic anti-and multi-resonance in overdamped monostable systems.

We have analyzed data from the flat-spectrum radio quasar PKS 1510-089 collected over a period of eight years from 2008 August to 2016 December with the Fermi-LAT. We have identified several flares of this highly variable source, studied their temporal and spectral properties in detail, and compared with previous works on flares of PKS 1510-089. Five major flares and a few subflares or substructures have been identified in our study. The fastest variability time is found to be 1.30 ± 0.18 hr between MJD 55852.063 and 55852.188, where we estimate the minimum size of the emission region to be 4.85 × 1015 cm. In most of the flares, the spectral energy distributions are better fitted with a log-parabolic distribution compared to a simple power law or a power law with exponential cutoffs. This has strong physics implications regarding the nature of the high-energy gamma-ray emission region.

Using six years of spectroscopic monitoring of the luminous quasar HE 0435-4312 (z = 1.2231) with the Southern African Large Telescope, in combination with photometric data (CATALINA, OGLE, SALTICAM, and BMT), we determined a rest-frame time delay of days between the Mg ii broad-line emission and the ionizing continuum using seven different time-delay inference methods. Time-delay artifact peaks and aliases were mitigated using the bootstrap method and prior weighting probability function, as well as by analyzing unevenly sampled mock light curves. The Mg ii emission is considerably variable with a fractional variability of ∼5.4%, which is comparable to the continuum variability (∼4.8%). Because of its high luminosity (L 3000 = 1046.4 erg s−1), the source is beneficial for a further reduction of the scatter along the Mg ii-based radius–luminosity relation and its extended versions, especially when the highly accreting subsample that has an rms scatter of ∼0.2 dex is considered. This opens up the possibility of using the high-accretor Mg ii-based radius–luminosity relation for constraining cosmological parameters. With the current sample of 27 reverberation-mapped sources, the best-fit cosmological parameters (Ωm, ΩΛ) = (0.19; 0.62) are consistent with the standard cosmological model within the 1σ confidence level.