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We present the results of optical photometric observations of three extreme TeV blazars, 1ES 0229+200, 1ES 0414+009, and 1ES 2344+514, taken with two telescopes (1.3 m Devasthal Fast Optical Telescope, and 1.04 m Sampuranand Telescope) in India and two (1.4 m Milanković telescope and 60 cm Nedeljković telescope) in Serbia during 2013–2019. We investigated their flux and spectral variability on diverse time-scales. We examined a total of 36 intraday R -band light curves of these blazars for flux variations using the power-enhanced F -test and the nested ANOVA test. No significant intraday variation was detected on 35 nights, and during the one positive detection the amplitude of variability was only 2.26 per cent. On yearly time-scales, all three blazars showed clear flux variations in all optical wavebands. The weighted mean optical spectral index (α BR ), calculated using B − R colour indices, for 1ES 0229+200 was 2.09 ± 0.01. We also estimated the weighted mean optical spectral indices of 0.67 ± 0.01 and 1.37 ± 0.01 for 1ES 0414+009, and 1ES 2344+514, respectively, by fitting a single power law (F ν ∝ ν−α) in their optical (VRI) spectral energy distributions. A bluer-when-brighter trend was only detected in the blazar 1ES 0414+009. We briefly discuss different possible physical mechanisms responsible for the observed flux and spectral changes in these blazars on diverse time-scales. [ABSTRACT FROM AUTHOR]

We present the results of photometric observations of three TeV blazars, 3C 66A, S4 0954+658, and BL Lacertae (BL Lac), during the period 2013–2017. Our extensive observations were performed in a total of 360 nights which produced ∼6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy time-scales. We also examine the optical spectral energy distributions of these blazars, which are crucial in understanding the emission mechanism of long-term variability in blazars. All three TeV blazars exhibited strong flux variability during our observations. The colour variations are mildly chromatic on long time-scales for two of them. The nature of the long-term variability of 3C 66A and S4 0954+658 is consistent with a model of a non-thermal variable component that has a continuous injection of relativistic electrons with power-law distributions around 4.3 and 4.6, respectively. However, the long-term flux and colour variability of BL Lac suggests that these can arise from modest changes in velocities or viewing angle toward the emission region, leading to variations in the Doppler boosting of the radiation by a factor of ∼1.2 over the period of these observations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present the results of extensive multiband intranight optical monitoring of BL Lacertae during 2010-2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them, observations were done quasi-simultaneously in B,V,R and I bands (totalling 113 light curves), with an average sampling interval of around 8 min. BL Lacertae showed significant variations on hour-like time-scales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states, deviates from the linear trend suggesting different jet components contributing to the emission at different times. [ABSTRACT FROM AUTHOR]

ABSTRACT We report results from a one-week multiwavelength campaign to monitor the BL Lacertae object (BL Lac) S5 0716+714 (on 2009 December 9-16). Nine ground-based telescopes at widely separated longitudes and one space-based telescope aboard the Swift satellite collected optical data. Radio data were obtained from the Effelsberg and Urumqi observatories and X-ray data from Swift. In the radio bands, the source shows rapid [∼(0.5-1.5) d] intraday variability with peak amplitudes of up to ∼10 per cent. The variability at 2.8 cm leads by about 1 d the variability at 6 and 11 cm. This time lag and more rapid variations suggest an intrinsic contribution to the source's intraday variability at 2.8 cm, while at 6 and 11 cm, interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ∼0.8 mag were detected in the V, R and I bands. The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 d time-scale, favouring reprocessed inverse Compton over synchrotron radiation in this band. The characteristic variability time-scales in radio and optical bands are similar. A quasi-periodic variation of 0.9-1.1 d in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical bands are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical) and is consistent with a high jet opacity and a compact synchrotron component peaking at ≃100 GHz in an ongoing very prominent flux-density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution, including γ-ray data from the Fermi satellite. We obtain lower limits for the relativistic Doppler boosting of δ ≥ 12-26, which for a BL Lac-type object is remarkably high. [ABSTRACT FROM AUTHOR]

Context. Rapid variations in optical flux are seen in many quasars and all blazars. The amount of variability in different classes of active galactic nuclei has been studied extensively but many questions remain unanswered. Aims. We present the results of a long-term programme to investigate the intra-night optical variability (INOV) of powerful flat spectrum radio core-dominated quasars (CDQs), with a focus on probing the relationship of INOV to the degree of optical polarization. Methods. We observed a sample of 16 bright CDQs showing strong broad optical emission lines and consisting of both high and low optical polarization quasars (HPCDQs and LPCDQs). In this first systematic study of its kind, we employed the 104-cm Sampurnanand telescope, the 201-cm Himalayan Chandra telescope and the 200-cm IUCAA-Girawali Observatory telescope, to carry out R-band monitoring on a total of 47 nights. Using the CCD as an N-star photometer to densely monitor each quasar for a minimum duration of about 4 h per night, INOV exceeding ∼1-2 per cent could be reliably detected. Combining these INOV data with those taken from the literature, after ensuring conformity with the basic selection criteria we adopted for the 16 CDQs monitored by us, we were able to increase the sample size to 21 CDQs (12 LPCDQs and 9 HPCDQs) monitored on a total of 73 nights. Results. As the existence of a prominent flat-spectrum radio core signifies that strong relativistic beaming is present in all these CDQs, the definitions of the two sets differ primarily in fractional optical polarization, with the LPCDQs showing a very low median Pop ≈ 0.4 per cent. Our study yields an INOV duty cycle (DC) of ∼28 per cent for the LPCDQs and ∼68 percent for HPCDQs. If only strong INOV with fractional amplitude above 3 per cent is considered, the corresponding DCs are ∼7 per cent and ∼40 per cent, respectively. Conclusions. From this strong contrast between the two classes of luminous, relativistically beamed quasars, it is apparent that relativistic beaming is normally not a sufficient condition for strong INOV and a high optical polarization is the other necessary condition. Moreover, the correlation is found to persist for many years after the polarization measurements were made. Some possible implications of this result are pointed out, particularly in the context of the recently detected rapid Υ-ray variability of blazars. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the results of our study of spectral energy distributions (SEDs) of a sample of 10 low- to intermediate-synchrotron-peaked blazars. We investigate some of the physical parameters most likely responsible for the observed short-term variations in blazars. To do so, we focus on the study of changes in the SEDs of blazars corresponding to changes in their respective optical fluxes. We model the observed spectra of blazars from radio to optical frequencies using a synchrotron model that entails a log-parabolic distribution of electron energies. A significant correlation among the two fitted spectral parameters ( a, b) of log-parabolic curves and a negative trend among the peak frequency and spectral curvature parameter, b, emphasize that the SEDs of blazars are fitted well by log-parabolic curves. On considering each model parameter that could be responsible for changes in the observed SEDs of these blazars, we find that changes in the jet Doppler factors are most important. [ABSTRACT FROM AUTHOR]

In order to address a currently much debated issue, we ate carrying out a systematic search for intra-night optical micro-variability in major classes of powerful (M[subv] ≥ -23.0 mag) active galactic nuclei (AGNs). Our sample includes 7 sets of AGNs, covering narrow redshift bins across the range 0.18 ≤ z ≤ 2.2 . Each set consists of a radio-quiet quasar, a radio lobe-dominated quasar, a BL Lac object and/or a radio core-dominated quasar (blazar), all having m[subv] ∼ 16 mag. The observations were made primarily using the 104-cm telescope of State Observatory, Nainital. The monitoring completed so far (∼ 90 nights) clearly indicates that, compared to the other AGN types, BL Lacs are distinctly more prone to exhibit intra-night optical variations. [ABSTRACT FROM AUTHOR]