arXiv subjects

R. Chanishvili

Publications and source records attributed to R. Chanishvili.

4 recordsLinked to original sources

arXiv2019

Characterizing optical variability of OJ 287 in 2016 - 2017

We report on a recent multi-band optical photometric and polarimetric observational campaign of the blazar OJ 287 which was carried out during September 2016 -- December 2017. We employed nine telescopes in Bulgaria, China, Georgia, Japan, Serbia, Spain and the United States. We collected over 1800 photometric image frames in BVRI bands and over 100 polarimetric measurements over ~175 nights. In 11 nights with many quasi-simultaneous multi-band (V, R, I) observations, we did not detect any genuine intraday variability in flux or color. On longer timescales, multiple flaring events were seen. Large changes in color with respect to time and in a color--magnitude diagram were seen, and while only a weak systematic variability trend was noticed in color with respect to time, the color--magnitude diagram shows a bluer-when-brighter trend. Large changes in the degree of polarization, and substantial swings in the polarization angle were detected. The fractional Stokes parameters of the polarization showed a systematic trend with time in the beginning of these observations, followed by chaotic changes and then an apparently systematic variation at the end. These polarization changes coincide with the detection and duration of the source at very high energies as seen by VERITAS. The spectral index shows a systematic variation with time and V-band magnitude. We briefly discuss possible physical mechanisms that could explain the observed flux, color, polarization, and spectral variability.

astro-ph.HE↗

arXiv2016

Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 -- 2016

We present recent optical photometric observations of the blazar OJ 287 taken during September 2015 -- May 2016. Our intense observations of the blazar started in November 2015 and continued until May 2016 and included detection of the large optical outburst in December 2016 that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of 9 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 March 2016. In addition, we investigated multi-band flux variations, colour variations, and spectral changes in the blazar on diverse timescales 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.

astro-ph.HE↗

arXiv2016

Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales

Here we report on the results of the WEBT photo-polarimetric campaign targeting the blazar S5~0716+71, organized in March 2014 to monitor the source simultaneously in BVRI and near IR filters. The campaign resulted in an unprecedented dataset spanning $\sim 110$\,h of nearly continuous, multi-band observations, including two sets of densely sampled polarimetric data mainly in R filter. During the campaign, the source displayed pronounced variability with peak-to-peak variations of about $30\%$ and "bluer-when-brighter" spectral evolution, consisting of a day-timescale modulation with superimposed hourlong microflares characterized by $\sim 0.1$\,mag flux changes. We performed an in-depth search for quasi-periodicities in the source light curve; hints for the presence of oscillations on timescales of $\sim 3$\,h and $\sim 5$\,h do not represent highly significant departures from a pure red-noise power spectrum. We observed that, at a certain configuration of the optical polarization angle relative to the positional angle of the innermost radio jet in the source, changes in the polarization degree led the total flux variability by about 2\,h; meanwhile, when the relative configuration of the polarization and jet angles altered, no such lag could be noted. The microflaring events, when analyzed as separate pulse emission components, were found to be characterized by a very high polarization degree ($> 30\%$) and polarization angles which differed substantially from the polarization angle of the underlying background component, or from the radio jet positional angle. We discuss the results in the general context of blazar emission and energy dissipation models.

astro-ph.HE↗

arXiv2014

Nature and dynamics of overreflection of Alfven waves in MHD shear flows

Our goal is to gain new insights into the physics of wave overreflection phenomenon in MHD nonuniform/shear flows changing the existing trend/approach of the phenomenon study. The performed analysis allows to separate from each other different physical processes, grasp their interplay and, by this way, construct the basic physics of the overreflection in incompressible MHD flows with linear shear of mean velocity, ${\bf U}_0=(Sy,0,0)$, that contain two different types of Alfv${\rm \acute{e}}$n waves. These waves are reduced to pseudo- and shear shear-Alfv${\rm \acute{e}}$n waves when wavenumber along $Z$-axis equals zero (i.e., when $k_z=0$). Therefore, for simplicity, we labelled these waves as: P-Alfv${\rm \acute{e}}$n and S-Alfv${\rm \acute{e}}$n waves (P-AWs and S-AWs). We show that: (1) the linear coupling of counter-propagating waves determines the overreflection, (2) counter-propagating P-AWs are coupled with each other, while counter-propagating S-AWs are not coupled with each other, but are asymmetrically coupled with P-AWs; S-AWs do not participate in the linear dynamics of P-AWs, (3) the transient growth of S-AWs is somewhat smaller compared with that of P-AWs, (4) the linear transient processes are highly anisotropic in wave number space, (5) the waves with small streamwise wavenumbers exhibit stronger transient growth and become more balanced, (6) maximal transient growth (and overreflection) of the wave energy occurs in the two-dimensional case -- at zero spanwise wavenumber. To the end, we analyze nonlinear consequences of the described anisotropic linear dynamics -- they should lead to an anisotropy of nonlinear cascade processes significantly changing their essence, pointing to a need of revisiting the existing concepts of cascade processes in MHD shear flows.

physics.plasm-ph↗