A. Strigachev

Quasi-simultaneous two-band optical variability of the blazars 1ES 1959+650 and 1ES 2344+514

We report the results of quasi-simultaneous two-filter optical monitoring of two high-energy peaked blazars, 1ES 1959+650 and 1ES 2344+514, to search for microvariability and short-term variability (STV). We carried out optical photometric monitoring of these sources in an alternating sequence of B and R passbands, and have 24 and 19 nights of new data for these two sources, respectively. No genuine microvariability (intranight variability) was detected in either of these sources. This non-detection of intranight variations is in agreement with the conclusions of previous studies that high-energy peaked BL Lacs are intrinsically less variable than low-energy peaked BL Lacs in the optical bands. We also report the results of STV studies for these two sources between 2009 July and 2010 August. Genuine STV is found for the source 1ES 1959+650 but not for 1ES 2344+514. We briefly discuss possible reasons for the difference between the intranight variability behaviour of high- and low-energy peaked blazars.

The 72-h WEBT microvariability observation of blazar S5 0716+714 in 2009

Context. The international Whole Earth Blazar Telescope (WEBT) consortium planned and carried out three days of intensive micro-variability observations of S5 0716+714 from February 22, 2009 to February 25, 2009. This object was chosen due to its bright apparent magnitude range, its high declination, and its very large duty cycle for micro-variations. Aims. We report here on the long continuous optical micro-variability light curve of 0716+714 obtained during the multi-site observing campaign during which the Blazar showed almost constant variability over a 0.5 mag range. The resulting light curve is presented here for the first time. Observations from participating observatories were corrected for instrumental differences and combined to construct the overall smoothed light curve. Methods. Thirty-six observatories in sixteen countries participated in this continuous monitoring program and twenty of them submitted data for compilation into a continuous light curve. The light curve was analyzed using several techniques including Fourier transform, Wavelet and noise analysis techniques. Those results led us to model the light curve by attributing the variations to a series of synchrotron pulses. Results. We have interpreted the observed microvariations in this extended light curve in terms of a new model consisting of individual stochastic pulses due to cells in a turbulent jet which are energized by a passing shock and cool by means of synchrotron emission. We obtained an excellent fit to the 72-hour light curve with the synchrotron pulse model.

Multiband optical variability of the blazar S5 0716+714 in outburst state during 2014–2015

We analyzed the multi-band optical behaviour of the BL Lacertae object, S5 0716+714, during its outburst state from 2014 November - 2015 March. We took data on 23 nights at three observatories, one in India and two in Bulgaria, making quasi-simultaneous observations in B, V, R, and I bands. We measured multi-band optical fluxes, colour and spectral variations for this blazar on intraday and short timescales. The source was in a flaring state during the period analyzed and displayed intense variability in all wavelengths. R band magnitude of 11.6 was attained by the target on 18 Jan 2015, which is the brightest value ever recorded for S5 0716+714. The discrete correlation function method yielded good correlation between the bands with no measurable time lags, implying that radiation in these bands originate from the same region and by the same mechanism. We also used the structure function technique to look for characteristic timescales in the light curves. During the times of rapid variability, no evidence for the source to display spectral changes with magnitude was found on either of the timescales. The amplitude of variations tends to increase with increasing frequency with a maximum of

The nature of the intra‐night optical variability in blazars

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Short‐term optical variability of high‐redshift quasi‐stellar objects

22% seen during flaring states in B band. A mild trend of larger variability amplitude as the source brightens was also found. We found the duty cycle of our source during the analyzed period to be

Nature of intranight optical variability of BL Lacertae

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Intranight variability of 3C 454.3 during its 2010 November outburst

90%. We also investigated the optical spectral energy distribution of S5 0716+714 using B, V, R, and I data points for 21 nights. We briefly discuss physical mechanisms most likely responsible for its flux and spectral variations.

Multicolour observations of nearby visual double stars. New CCD measurements and orbits

In this paper we present the results of a short-term optical monitoring program of 13 blazars. The objects were monitored mostly in the R band for a total of ∼160 h between 2006 and 2011. We study the nature of the short-term variations and show that most of them could be described as slow, smooth and (almost) linear changes of up to ∼0.1 mag h −1 , but that many objects show no short-term variations at all. In fact, we found only a ∼2 per cent chance of observing variability of more than 0.1 mag h −1 for the sample we observed. Hints of quasiperiodic oscillations at very low-amplitude levels are also found for some objects. We briefly discuss some of the possible mechanisms for generating the intra-night variability and the quasi-periodic oscillations.

Multiband optical variability of three TeV blazars on diverse time-scales

In this paper we present the results of a search for short-term variability in the optical band of selected high-luminosity, high-redshift radio-quiet quasars. Each quasar has been monitored typically for 2-4 h with a time resolution of 2-5 min and a photometric accuracy of about 0.01-0.02 mag. As a result of the significant redshift (z > 2), the covered wavelength range falls into the ultraviolet region (typically 1500-2500 A). We have found no statistical evidence for any continuum variations larger than 0.01-0.02 mag for any of the monitored objects. Our results suggest that the presence of a short-term variability in radio-quiet quasars is unlikely even in the ultraviolet region, contrary to reports by other authors. This conclusion holds true at least for high-luminosity (large black hole mass and accretion rate?) objects. The results are consistent with the idea that significant short-term (less than 1 h) variations in active galactic nuclei, where observed, should be attributed primarily to processes in a relativistic jet.

Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT campaign and its extension.

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.