arXiv subjects

Sergey S. Savchenko

Publications and source records attributed to Sergey S. Savchenko.

41 records · Page 3Linked to original sources

arXiv2020

New X-shaped bulge photometric model as a tool for measuring B/PS bulges and their X-structures in photometric studies

Recent orbital studies of 3D bar structure in various numerical and analytical models show that X-structures that reside in boxy/peanut-shaped (B/PS) bulges are not delineated by some specific type of orbits, but are natural parts of them and formed by the same orbits that constitute such bulges. This implies that to accurately account for B/PS bulges and their X-structures in photometric studies, one needs the photometric model of B/PS bulge that includes an X-structure as its natural part. To find such a model, we considered a self-consistent numerical galaxy model where a typical B/PS bulge arises. Using spectral characteristics of particle-"stars", we decomposed the galaxy model onto the bar and non-bar components. We used the extracted 3D bar component to find an appropriate B/PS bulge photometric model, which can account for X-structures residing in such bulges. The resulted B/PS bulge photometric model has a truncated 2D Sersic profile with truncations introduced above (in the upper half-plane) and below (in the bottom half-plane) the rays of X-structures. We applied this model to represent B/PS bulges of various numerical models and some real galaxies. The comparison with previous works revealed that there are systematic shifts between the X-structure parameters of the same galaxies measured within the different approaches. We found that the geometric parameters of X-structures of real and modelled galaxies are consistent with each other if we measure them using our new model.

astro-ph.GA↗

arXiv2019

Multiband optical flux density and polarization microvariability study of optically bright blazars

We present the results of flux density, spectral index, and polarization intra-night monitoring studies of a sample of eight optically bright blazars, carried out by employing several small to moderate aperture (0.4\,m to 1.5\,m diameter) telescopes fitted with CCDs and polarimeters located in Europe, India, and Japan. The duty cycle of flux variability for the targets is found to be $\sim 45$ percent, similar to that reported in earlier studies. The computed two-point spectral indices are found to be between 0.65 to 1.87 for our sample, comprised of low- and intermediate frequency peaked blazars, with one exception; they are also found to be statistically variable for about half the instances where `confirmed' variability is detected in flux density. In the analysis of the spectral evolution of the targets on hourly timescale, a counter-clockwise loop (soft-lagging) is noted in the flux-spectral index plane on two occasions, and in one case a clear spectral flattening with the decreasing flux is observed. In our data set, we also observe a variety of flux-polarization degree variability patterns, including instances with a relatively straightforward anti-correlation, correlation, or counter-clockwise looping. These changes are typically reflected in the flux-polarization angle plane: the anti-correlation between the flux and polarization degree is accompanied by an anti-correlation between the polarization angle and flux, while the counter-clockwise flux-PD looping behaviour is accompanied by a clockwise looping in the flux-polarization angle representation. We discuss our findings in the framework of the internal shock scenario for blazar sources.

astro-ph.HE↗

arXiv2019

Inhomogeneous Broadening of the Exciton Band in Optical Absorption Spectra of InP/ZnS Nanocrystals

In this work, we have simulated the processes of broadening the first exciton band in optical absorption spectra (OA) for InP/ZnS ensembles of colloidal quantum dots (QDs). A phenomenological model has been proposed that takes into account the effects of the exciton-phonon interaction and allows one to analyze the influence of the static and dynamic types of atomic disorder on the temperature changes in the spectral characteristics in question. To vary the degree of static disorder in the model system, we have used a parameter $δ$, which characterizes the QD dispersion in size over the ensemble. Also, we have calculated the temperature shifts of the maxima and changes in the half-width for the exciton peaks in single nanocrystals ($δ$ = 0), as well as for the integrated OA bands in the QD ensembles with different values of $δ$ = 0.6 - 17%. The simulation results and the OA spectra data measured for InP/ZnS nanocrystals of 2.1 nm ($δ$ = 11.1%) and 2.3 nm ($δ$ = 17.3%) are in good mutual agreement in the temperature range of 6.5 K - RT. It has been shown that the contribution of static disorder to the observed inhomogeneous broadening of the OA bands for the QDs at room temperature exceeds 90%. The computational experiments performed indicate that the temperature shift of the maximum for the integrated OA band coincides with that for the exciton peak in a single nanocrystal. In this case, a reliable estimate of the parameters of the fundamental exciton-phonon interaction can be made. Simultaneously, the values of the specified parameters, calculated from the temperature broadening of the OA spectra, can be significantly different from the true ones due to the effects of static atomic disorder in real QD ensembles.

cond-mat.mtrl-sci↗

arXiv2019

The June 2016 Optical and Gamma-Ray Outburst and Optical Micro-Variability of the Blazar 3C454.3

The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The blazar was observed in the optical $R$ band by several ground-based telescopes in photometric and polarimetric modes, at $γ$-ray frequencies by the \emph{Fermi}\ Large Area Telescope, and at 43 GHz with the Very Long Baseline Array. The maximum flux density was observed on 2016 June 24 at both optical and $γ$-ray frequencies, reaching $S^\mathrm{max}_\mathrm{opt}=18.91\pm0.08$ mJy and $S_γ^\mathrm{max} =22.20\pm0.18\times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, respectively. The June 2016 outburst possessed a precipitous decay at both $γ$-ray and optical frequencies, with the source decreasing in flux density by a factor of 4 over a 24-hour period in $R$ band. Intraday variability was observed throughout the outburst, with flux density changes between 1 and 5 mJy over the course of a night. The precipitous decay featured statistically significant quasi-periodic micro-variability oscillations with an amplitude of $\sim 2$-$3\%$ about the mean trend and a characteristic period of 36 minutes. The optical degree of polarization jumped from $\sim3\%$ to nearly 20\% during the outburst, while the position angle varied by $\sim120\degr$. A knot was ejected from the 43 GHz core on 2016 Feb 25, moving at an apparent speed $v_\mathrm{app}=20.3c\pm0.8c$. From the observed minimum timescale of variability $τ_\mathrm{opt}^\mathrm{min}\approx2$ hr and derived Doppler factor $δ=22.6$, we find a size of the emission region $r\lesssim2.6\times10^{15}$ cm. If the quasi-periodic micro-variability oscillations are caused by periodic variations of the Doppler factor of emission from a turbulent vortex, we derive a rotational speed of the vortex $\sim0.2c$.

astro-ph.HE↗

arXiv2016

Galaxies with conspicuous optical warps

In this paper, we present results of a photometric and kinematic study for a sample of 13 edge-on spiral galaxies with pronounced integral-shape warps of their stellar discs. The global structure of the galaxies is analyzed on the basis of the Sloan Digital Sky Survey (SDSS) imaging, in the g, r and i passbands. Spectroscopic observations are obtained with the 6-m Special Astrophysical Observatory telescope. In general, galaxies of the sample are typical bright spiral galaxies satisfying the Tully-Fisher relation. Most of the galaxies reside in dense spatial environments and, therefore, tidal encounters are the most probable mechanism for generating their stellar warps. We carried out a detailed analysis of the galaxies and their warps and obtained the following main results: (i) maximum angles of stellar warps in our sample are about 20°; (ii) warps start, on average, between 2 and 3 exponential scale lengths of a disc; (iii) stronger warps start closer to the center, weak warps start farther; (iv) warps are asymmetric, with the typical degree of asymmetry of about several degrees (warp angle); (v) massive dark halo is likely to preclude the formation of strong and asymmetric warps.

astro-ph.GA↗