Blazars show optical and γ-ray flux variations that are generally correlated, although there are exceptions. Here we present anomalous behaviour seen in the blazar 3C 454.3 based on an analysis of quasi-simultaneous data at optical, ultraviolet, X-ray, and γ-ray energies, spanning about 9 yr from 2008 August to 2017 February. We have identified four time intervals (epochs), A, B, D, and E, when the source showed large-amplitude optical flares. In epochs A and B the optical and γ-ray flares are correlated, while in D and E corresponding flares in γ-rays are weak or absent. In epoch B the degree of optical polarization strongly correlates with changes in optical flux during a short-duration optical flare superimposed on one of long duration. In epoch E the optical flux and degree of polarization are anticorrelated during both the rising and declining phases of the optical flare. We carried out broad-band spectral energy distribution (SED) modelling of the source for the flaring epochs A,B, D, and E, and a quiescent epoch, C. Our SED modelling indicates that optical flares with absent or weak corresponding γ-ray flares in epochs D and E could arise from changes in a combination of parameters, such as the bulk Lorentz factor, magnetic field, and electron energy density, or be due to changes in the location of the γ-ray-emitting regions.
Blazars are known to show flux variations over a range of energies from low energy radio to high energy γ-rays. Cross-correlation analysis of the optical and γ-ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical-GeV connection in four flat spectrum radio quasars (FSRQs) by a systematic investigation of their long term optical and γ-ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510−089 and CTA 102 we noticed different behaviours between the optical and GeV flux variations. We found instances when (i) the optical and GeV flux variations are closely correlated (ii) there are optical flares without γ-ray counterparts and (iii) γ-ray flares without optical counterparts. To understand these diverse behaviours, we carried out broad band spectral energy distribution (SED) modelling of the sources at different epochs using a one-zone leptonic emission model. The optical-UV emission is found to be dominated by emission from the accretion disk in the sources PKS 1510−089, CTA 102 and 3C 273, while in 3C 279, the synchrotron radiation from the jet dominates the optical-UV emission. Our SED analysis indicates that (i) correlated optical and γ-ray flux variations are caused by changes in the bulk Lorentz factor (Γ), (ii) γ-ray flares without optical counterparts are due to increase in Γ and/or the electron energy density and (iii) an optical flare without γ-ray counterpart is due to increase in the magnetic field strength.
We present the results of our broadband study of the γ-ray emitting narrow line Seyfert 1 (NLSy1) galaxy SBS 0846+513 (z = 0.585). This includes multi-band flux variations, γ-ray spectral analysis, broad band spectral energy distribution (SED) modeling, and intranight optical variability (INOV) observations carried over 6 nights between 2012 November and 2013 March using the 2 m Himalayan Chandra Telescope and the 1.3 m telescope at Devasthal. Multiple episodes of flaring activity are seen in the γ-ray light curve of the source which are also reflected in the observations at lower frequencies. A statistically significant curvature is noticed in the seven years averaged γ-ray spectrum, thus indicating its similarity with powerful flat spectrum radio quasars (FSRQs). Modeling the SEDs with a one zone leptonic emission model hints the optical-UV spectrum to be dominated by synchrotron radiation, whereas, inverse Compton scattering of broad line region photons reproduces the γ-ray part of the SEDs. The source was found to be variable on all the six nights of optical observations with a variation of ∼0.3 magnitude within a single night, coinciding with a high γ-ray activity state. The observed large amplitude INOV clearly indicates the presence of a closely aligned beamed relativistic jet in SBS 0846+513. Our broadband study supports the recent claims in literature that γ-ray emitting NLSy1 galaxies are similar to blazars and constitute the low black hole mass counterparts to FSRQs.
Study of the polarization behaviour in blazars is a powerful tool to discern the role of the magnetic field in the variable emission process in their relativistic jets. We present here the results of our systematic investigation on the correlation between optical flux and polarization variations for eight flat-spectrum radio quasars on various time-scales using data from the Steward Observatory that covers a period of ∼10 years. On long time-scales (∼several months), from a total of 79 observing cycles, we found a significant positive correlation between optical flux and optical polarization degree (PD) in 34 observing cycles, a negative correlation in three cycles, and no correlation in 42 cycles. On short time-scales (∼few days), in 47 out of a total of 55 epochs we found a positive correlation between optical flux and PD, while for the remaining eight epochs an anticorrelation was detected between the two quantities. Moreover, we noticed a significant positive correlation between optical and γ-ray fluxes in 14 epochs, and a negative correlation between the two in one epoch. While the observed optical flux changes fit the shock-in-jet model well, the observed changes in PD are not explainable by changes in the power-law spectral index of the relativistic electrons in the jet. Instead, the observed varied correlations between optical flux and PD could be due to multizone emission regions or the enhanced flux coinciding with the emergence of a new emission knot with its magnetic field either aligned or misaligned with the large-scale magnetic field.
We used the data from the Fermi Gamma-ray Space Telescope to characterise the γ-ray flux variability of blazars on month-like time scales. Our sample consists of 1120 blazars of which 481 are flat spectrum radio quasars (FSRQs) and 639 are BL Lac objects (BL Lacs). We generated monthly binned light curves of our sample for a period of approximately nine years from 2008 August to 2017 December and quantified variability by using excess variance (F var ). On month-like time scales, 371/481 FSRQs are variable ( 80%), while only about 50% (304/639) of BL Lacs are variable. This suggests that FSRQs are more variable than BL Lac objects. We find a mean F var of 0.55 ± 0.33 and 0.47 ± 0.29 for FSRQs and BL Lacs respectively. Large F var in FSRQs is also confirmed from the analysis of the ensemble structure function. By Dividing our sample of blazars based on the position of the synchrotron peak in their broad-band spectral energy distribution, we find that the low synchrotron peaked (LSP) sources have the largest mean F var value of 0.54 ± 0.32 while the intermediate synchrotron peaked (ISP) and high synchrotron peaked (HSP) sources have mean F var values of 0.45 ± 0.25 and 0.47 ± 0.33 respectively. On month-like time scales, we find FSRQs to show a high duty cycle (DC) of variability of 66% relative to BL Lacs that show a DC of 36%. We find that both the F var and time scale of variability (τ) do not correlate with M BH . We note that F var is found to be weakly correlated with Doppler factor (δ) and τ is also weakly correlated with δ. Most of the sources in our sample have τ of the order of days, which might be related to processes in the jet. We find marginal difference in the distribution of τ between FSRQs and BL Lacs.
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