Analysis of the color variability of ON 231 in 1994–2002

We analyze multicolor observations of the blazar ON 231 obtained during coordinated observations in 1994–2002. On average, the spectral energy distribution of the variable component in the optical range remains constant, and can be represented by the power law F _ν ∼ ν ^−0.85. Since the radiation of the blazar is strongly polarized, there is no doubt that the variable emission that is responsible for the activity of the blazar is synchrotron radiation. There are small but significant season-to-season variations in the spectral index.     

Analysis of Multicolor Observations of 3C 66A in 1993-1998

The results of multicolor observations of the blazar 3C 66A, obtained in the course of the project OJ-94, are analyzed. It is established that the energy distribution of the variable source in the optical and IR ranges can be represented, on the average, by a power law: F ^-0.92. Since the blazar's emission is strongly polarized, there is no doubt of the synchrotron nature of the variable source responsible for the blazar's activity.     

Color variations of the blazar 3C 454.3 in 2004–2006

We present an analysis of multicolor (U BV RI JH K) observations of the blazar 3C 454.3 made in 2004–2006. We used the light curves compiled at the Turin Observatory from coordinated observations in the framework of the WEBT program. We consider color variations in two time intervals, when an unprecedented strong outburst occurred (2004–2005), and when the object was in a post-eruptive state and a low-amplitude brightness increase was observed (2006). The spectral energy distribution (SED) of the variable component remained the same within each of these intervals, but differed between them. In both cases, this SED followed a power law after correction for extinction, suggesting the variable component has a synchrotron nature. We conclude that the variations in the optical and IR were due to the same variable source. The object’s unusual color behavior (the brighter, the redder) was due to an increasing contribution from a variable component that was redder than the constant component (big blue bump).     

Photopolarimetry of BL Lac

This paper presents results of our monitoring of BL Lac at the 2.56 m Nordic Optical Telescope in La Palma and at the 0.91 m telescope at National Astronomical Observatory in Dodaira, Japan. On La Palma we used aUBVRI-photopolarimeter which gives truly simultaneous observations in all five colour bands. At Dodaira we used a photopolarimeter which gives simultaneously photometry atB, V andR-bands and polarimetry atV-band. We have observations of BL Lac during flare behaviour and during these times we have observed a clear rotation of the polarization position angle. During these flares the polarization level has also been strongly variable and the object has lost its preferred direction of the polarization position angle. The reason for this behaviour remains still unclear, but there are two possible explanations: shocks in the helical magnetic field in the jet pointing nearly towards us or polarized synchrotron flares rotating within the accretion disk (seen nearly face on) of the supermassive central black hole.     

3C 66A: Variability in 2007–2015

The results of photometric (BV RIJHK) and polarimetric (R)monitoring of the blazar 3C 66A performed at the St. Petersburg State University and the Central AstronomicalObservatory of the Russian Academy of Sciences in 2007–2015, radio observations performed by the Boston University team with the Very Long Baseline Array at 43 GHz, and a gamma-ray light curve based on observations with the Fermi SpaceObservatory are presented. Color variations of the object are studied. Changes in the optical spectral energy distribution are observed at some times, indicating the appearance and disappearance of individual variable sources. A variable source with a degree of polarization of 36% is identified, which is responsible for the polarization variations observed during one episode. The correlations between the variations in the different spectral ranges indicate that the optical and gamma-ray radiation originates near the radio core detected at 43 GHz. The presence of five superluminal components emerging from the core is detected.     

Optical variability of the blazar S4 0954+658 in 2008–2012

The results of multi-color photometric and polarization observations of the blazar S4 0954+658 carried out mainly in the Astronomical Institute of St. Petersburg State University and the Central Astronomical Observatory of the Russian Academy of Sciences in 2008–2012 are analyzed. Individual variable components that are responsible for the activity are distinguished; the power-law spectrum and high degree of polarization confirm that the emission is synchrotron radiation. Modeling the observed dependences between the parameters of the polarization and intensity is used to derive parameters of both the constant and the variable components of the radiation. The observed color variability (“the brighter, the bluer”) can be explained by the superposition of a red constant component and a bluer variable component with a constant relative spectral energy distribution.     

Simultaneous linear and circular polarization observations of blazars 3C 66A,OJ 287 and Markarian 421

We present the first ever simultaneous optical linear and circular polarization observations of blazars. These polarizations have been measured simultaneously inUBVRI-bands in three blazars; 3C 66A, OJ 287 and Markarian 421. Measured linear polarization in 3C 66A was the largest ever observed, at PR = 33.1 ± 0.5 per cent. In 3C 66A we detected small circular polarization in the other bands, exceptU. In OJ 287 we detected variable circular polarization in theU-band.     

Photopolarimetry of Seyfert galaxy MK 509

We present the first truly simultaneousUBVRI photopolarimetric observations of the Seyfert galaxy Markarian 509. The photometric light curves show small amplitude variations in timescales of days. These variations are largest in theU-band and smallest in theI-band, indicating that the variations are due to a decline in the synchrotron emission from the galactic nucleus. We measured a constant one per cent polarization. A small frequency dependence was observed in the polarization during some of the nights. No frequency depencdence was seen in the position angle. These observations indicate that Markarian 509 is similar to 3C 273, i.e., a blazar.     

Color variability of the blazar AO 0235+16

Multicolor (UBVRIJHK) observation of the blazar AO 0235+16 are analyzed. The light curves were compiled at the Turin Observatory from literature data and the results of observations obtained in the framework of the WEBT program (). The color variability of the blazar was studied in eight time intervals with a sufficient number of multicolor optical observations; J H K data are available for only one of these. The spectral energy distribution (SED) of the variable component remained constant within each interval, but varied strongly from one interval to another. After correction for dust absorption, the SED can be represented by a power law in all cases, providing evidence for a synchrotron nature of the variable component. We show that the variability at both optical and IR wavelengths is associated with the same variable source.     

Optical and IR monitoring of the BL Lac object S5 0716+714 from 2001–2004

Multicolor (BV RIJHK) observations of the BL Lac object S5 0716+714 carried out irom 2001–2004 indicate that the spectral energy distribution of the slowly varying component remained constant over the entire wavelength interval and over the three-year observation period. The distribution can be represented by the power law F _v ∼ v ^−1.12, providing evidence that this component has a synchrotron nature. The color characteristics of very rapid variability detected on several nights are the same as those for the slowly varying component. An analysis of published data on the color variability of the object for previous years indicates that the color characteristics of the slowly varying component are stable, independent of the time intervals considered and the characteristic variability time scales.