arXiv subjects

S. Ibryamov

Publications and source records attributed to S. Ibryamov.

At least 19 recordsLinked to original sources

arXiv2026

Multi-band optical photometric variability of the blazar OJ 287 from 2015 to 2025

We present the most densely sampled multi-band optical photometric observations of the peculiar BL Lacertae object OJ 287 from 2015 to 2025 with a focus on its optical activity on diverse timescales. We present a total of 2296, 10927, 11484, and 2982 data points in B, V, R, and I bands, respectively. The densely sampled observations allow us to keep track of the source evolution that it has exhibited since the start of the predicted major optical flaring activity at the end of 2015. The study reveals clear and persistent bluer when brighter trends in both the long-term and short-term variations. Different bands were cross-correlated with discrete correlation functions, which peak at zero lag, implying co-spatial emission. Using eight optical spectra in the low flux states of OJ 287 taken from 2017 October 21 to 2017 November 22, from Steward Observatory, we estimate the central black hole mass to be at least 3.89 $\times \ \rm{10}^{9} \ \rm{M}_{\odot}$ from the [O III] line width. The emission mechanism of the binary black hole blazar, and its possible implication in various aspects of multi-messenger astronomy are briefly discussed.

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arXiv2024

Multi-wavelength picture of the misaligned BL Lac object 3C 371

The BL Lac object 3C 371 is one of the targets that are regularly monitored by the Whole Earth Blazar Telescope (WEBT) Collaboration to study blazar variability on both short and long timescales. We aim to evaluate the long-term multiwavelength (MWL) behaviour of 3C 371, comparing it with the results derived for its optical emission in our previous study. For this, we make use of the multi-band campaigns organized by the WEBT Collaboration in optical and radio between January 2018 and December 2020, and of public data from Swift and Fermi satellites and the MOJAVE Very Large Interferometry programme. We evaluate the variability shown by the source in each band with the amplitude variability quantification, as well as possible interband correlation using the z-Discrete Correlation Function. We also present a deep analysis of the optical-UV, X-ray and $γ$-ray spectral variability. With the MOJAVE data we perform a kinematics analysis, looking for components propagating along the jet, calculating its kinematics parameters. This set of parameters is later used for the interpretation of the source MWL behaviour, modelling the broadband spectral energy distribution (SED) of the source with theoretical blazar emission scenarios.

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arXiv2024

Optical variability of the blazar 3C 371: from minute to year timescales

The BL Lac object 3C 371 has been observed by the Transiting Exoplanet Survey Satellite (\textit{TESS}) for approximately a year, between July 2019 and July 2020, with an unmatched 2-minute observing cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. These datasets allow for a thorough investigation of the variability of the source. The goal of this study is evaluating the optical variability of 3C 371. Taking advantage of the remarkable cadence of \textit{TESS} data, we aim to characterize the intra-day variability (IDV) displayed by the source and identify its shortest variability timescale. With this estimate, constraints on the size of the emitting region and black hole mass can be calculated. Moreover, WEBT data is used to investigate long-term variability (LTV), including understanding spectral behaviour of the source and the polarization variability. Based on the derived characteristics, information on the origin of the variability on different timescales is extracted. We evaluated the variability applying the variability amplitude tool that quantifies how variable the emission is. Moreover, we employed common tools like ANOVA (ANalysis Of VAariance) tests, wavelet and power spectral density (PSD) analyses to characterize the shortest variability timescales present in the emission and the underlying noise affecting the data. Short- and long-term colour behaviours have been evaluated to understand the spectral behaviour of the source. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of IDV and LTV were also studied with the aim of linking the flux variations to turbulent and/or accretion disc related processes.

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arXiv2023

Extreme photometric and polarimetric variability of blazar S4 0954+65 at its maximum optical and $γ$-ray brightness levels

In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and $γ$-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The WEBT data show rapid variability in both the degree and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle. All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic reconnection. The WEBT optical data are compared to $γ$-ray data from the Fermi satellite. These are analysed with both fixed and adaptive binning procedures. We show that the strong correlation between optical and $γ$-ray data without measurable delay assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates.

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arXiv2022

Multi-band Optical Variability of the TeV Blazar PG 1553+113 in 2019

We report the flux and spectral variability of PG 1553+113 on intra-night (IDV) to short-term timescales using BVRI data collected over 91 nights from 28 February to 8 November 2019 employing ten optical telescopes: three in Bulgaria, two each in India and Serbia, and one each in Greece, Georgia, and Latvia. We monitored the blazar quasi-simultaneously for 16 nights in the V and R bands and 8 nights in the V, R, I bands and examined the light curves (LCs) for intra-day flux and colour variations using two powerful tests: the power-enhanced F-test and the nested ANOVA test. The source was found to be significantly (> 99%) variable in 4 nights out of 27 in R-band, 1 out of 16 in V-band, and 1 out of 6 nights in I-band. No temporal variations in the colours were observed on IDV timescale. During the course of these observations the total variation in R-band was 0.89 mag observed. We also investigated the spectral energy distribution (SED) using B, V, R, and I band data. We found optical spectral indices in the range of 0.878+-0.029 to 1.106+-0.065 by fitting a power law to these SEDs of PG 1553+113. We found that the source follows a bluer-when-brighter trend on IDV timescales. We discuss possible physical causes of the observed spectral variability.

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arXiv2022

Long-term optical photometry of the PMS stars V2764 Ori and LkH$α$ 301 in the field of the McNeil's Nebula

In this paper, we present results from long-term $V(RI)_{c}$ photometric observations of the pre-main-sequence stars V2764 Ori and LkH$α$ 301, located in the field of the McNeil's Nebula within the Orion star-forming complex. Our observations were performed in the period from August 2004 to November 2021 with three telescopes and seven different types of CCD cameras. Photometric observations, especially concerning the long-term behavior of the stars, are missing in the literature. We present the first photometric monitoring for them, which cover 17 years. Our data indicate that the variability of both stars is typical for classical T Tauri stars.

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arXiv2020

Multi-Wavelength Variability of BL Lacertae Measured with High Time Resolution

In an effort to locate the sites of emission at different frequencies and physical processes causing variability in blazar jets, we have obtained high time-resolution observations of BL Lacertae over a wide wavelength range: with the \emph{Transiting Exoplanet Survey Satellite} (TESS) at 6,000-10,000 Å with 2-minute cadence; with the Neil Gehrels \emph{Swift} satellite at optical, UV, and X-ray bands; with the Nuclear Spectroscopic Telescope Array at hard X-ray bands; with the \emph{Fermi} Large Area Telescope at $γ$-ray energies; and with the Whole Earth Blazar Telescope for measurement of the optical flux density and polarization. All light curves are correlated, with similar structure on timescales from hours to days. The shortest timescale of variability at optical frequencies observed with TESS is $\sim 0.5$ hr. The most common timescale is $13\pm1$~hr, comparable with the minimum timescale of X-ray variability, 14.5 hr. The multi-wavelength variability properties cannot be explained by a change solely in the Doppler factor of the emitting plasma. The polarization behavior implies that there are both ordered and turbulent components to the magnetic field in the jet. Correlation analysis indicates that the X-ray variations lag behind the $γ$-ray and optical light curves by up to $\sim 0.4$ days. The timescales of variability, cross-frequency lags, and polarization properties can be explained by turbulent plasma that is energized by a shock in the jet and subsequently loses energy to synchrotron and inverse Compton radiation in a magnetic field of strength $\sim3$ G

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arXiv2020

Intra-night flickering of RS Ophiuchi: III. Modes of quasi-periods in the minute scale and their evolution

We study the photometric behavior of the recurrent nova RS Oph by 58 monitoring light curves (LCs), taken by 5 telescopes. All LCs show repeating time structures with some quasi-periods (QPs) in time scales from minutes to hours. In our previous work 97 QPs were detected in the LCs by local minimums of structure functions and local maximums of auto-correlation functions. The distribution of the QPs shows modes at 8, 13, 21, 30, 48 and 73 min, where the mode at 8 min is poorly unveiled. These modes follow a power function with base $1.55\approx 3/2$ with standard deviation 4.7%. This function predicts modes also at 5.3 and 3.5 min, which are not detected in the full MLCs. In the present work we analyze simple small parts from high resolution LCs. We confirm the QPs modes at 8.0, 5.3 and 3.5 min. Generally, we found 8 QP modes with regular logarithmic distribution in the time interval 3.5-73 min. We also show typical intra-night evolutions of QP modes in the minute scale -- sharp or gradual transitions from one QP mode to other. In the end we find that the parts of the LCs carry out the properties of the whole LCs at short time scale. This lead to two well pronounces dependences - between the range deviation and standard deviation of the LC, as well as between the quasi-period and the relevant level of the density function of the LC.

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arXiv2020

Multiwavelength behaviour of the blazar 3C279: decade-long study from $γ$-ray to radio

We report the results of decade-long (2008-2018) $γ$-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, $\it{Fermi}$ and $\it{Swift}$ data, as well as polarimetric and spectroscopic data. The X-ray and $γ$-ray light curves correlate well, with no delay > 3 hours, implying general co-spatiality of the emission regions. The $γ$-ray-optical flux-flux relation changes with activity state, ranging from a linear to a more complex dependence. The behaviour of the Stokes parameters at optical and radio wavelengths, including 43 GHz VLBA images, supports either a predominantly helical magnetic field or motion of the radiating plasma along a spiral path. Apparent speeds of emission knots range from 10 to 37c, with the highest values requiring bulk Lorentz factors close to those needed to explain $γ$-ray variability on very short time scales. The Mg II emission line flux in the `blue' and `red' wings correlates with the optical synchrotron continuum flux density, possibly providing a variable source of seed photons for inverse Compton scattering. In the radio bands we find progressive delays of the most prominent light curve maxima with decreasing frequency, as expected from the frequency dependence of the $τ=1$ surface of synchrotron self-absorption. The global maximum in the 86 GHz light curve becomes less prominent at lower frequencies, while a local maximum, appearing in 2014, strengthens toward decreasing frequencies, becoming pronounced at $\sim5$ GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet.

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arXiv2020

International observational campaign of the 2014 eclipse of EE Cep

Context. EE Cep is one of few eclipsing binary systems with a dark, dusty disk around an invisible object similar to ε Aur. The system is characterized by grey and asymmetric eclipses every 5.6 yr, with a significant variation in their photometric depth, ranging from ~ 0 m .5 to ~ 2 m .0. Aims. The main aim of the observational campaign of the EE Cep eclipse in 2014 was to test the model of disk precession (Galan et al. 2012). We expected that this eclipse would be one of the deepest with a depth of ~ 2 m .0. Methods. We collected multicolor observations from almost 30 instruments located in Europe and North America. This photometric data covers 243 nights during and around the eclipse. We also analyse the low- and high-resolution spectra from several instruments. Results. The eclipse was shallow with a depth of 0 m .71 in V-band. The multicolor photometry illustrates small color changes during the eclipse with a total amplitude of order ~ +0 m . 15 in B-I color index. The linear ephemeris for this system is updated by including new times of minima, measured from the three most recent eclipses at epochs E = 9, 10 and 11. New spectroscopic observations were acquired, covering orbital phases around the eclipse, which were not observed in the past and increased the data sample, filling some gaps and giving a better insight into the evolution of the H α and NaI spectral line profiles during the primary eclipse. Conclusions. The eclipse of EE Cep in 2014 was shallower than expected 0 m .71 instead of ~ 2 m . 0. This means that our model of disk precession needs revision.

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arXiv2019

Optical Variability of TeV Blazars on long time-scales

We present the results of photometric observations of three TeV blazars, 3C 66A, S5 0954+658 and BL Lacertae, during the period 2013--2017. Our extensive observations were performed in a total of 360 nights which produced $\sim$6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy timescales. 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 timescales for two of them. The nature of the long-term variability of 3C 66A and S5 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 ~1.2 over the period of these observations.

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arXiv2018

Intra-night flickering of RS Ophiuchi: I. Sizes and cumulative energies of time structures

We analyzed 29 pairs of time series in B and V bands of the recurrent nova RS Oph. The observations were carried out in 2008-2017 with duration 0.6 - 3.6 hours, with time resolution 0.5 - 3.3 min. We scanned digitally each series by data windows with various sizes Theta and derived two of the simplest fractal parameters for every Theta - standard deviation D and structural deviation S. Using the local minima of the structural function log S = f_S(log Theta) we unveiled 80 time structures, 42 in B band and 38 in V band, with time sizes 10-120 min. About 3/4 of the time sizes belong to the interval 10-40 min and about 1/4 lie in the interval 60-120 min. The respective cycles per day are 144-36 c/d and 24-15 c/d. On logarithmic scale, the distribution of the time sizes shows maximums at about 10, 21, 36 and 74 min. The 10 min flickering is poorly detectable in our series and we found the most widespread time structures (in about 1/5 of the cases) have time sizes about 21 min (about 69 c/d). Using the deviation function log D = f_D(log Theta) we estimated the relative cumulative energy (including the energy of the shorter structures in it), associated with the detected structure sizes, to be in the interval of the relative fluxes 2-11 %. The energies correlate weakly with the logarithms of the structure sizes, with correlation coefficients 0.60 and 0.57, under slope coefficients 0.04 and 0.03 in B and V band, respectively. The distributions of the energies occur bimodal, with maximums about 4% and 6% in B band, as well as about 3% and 5% in V band. The left and right modes of the distributions may be associated with the structure sizes 10 - 21 min and 37 - 74 min, respectively.

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arXiv2017

Multi-Band Intra-Night Optical Variability of BL Lacertae

We monitored BL Lacertae frequently during 2014 - 2016 when it was generally in a high state. We searched for intra-day variability for 43 nights using quasi-simultaneous measurements in the B, V, R, and I bands (totaling 143 light curves); the typical sampling interval was about eight minutes. On hour-like timescales, BL Lac exhibited significant variations during 13 nights in various optical bands. Significant spectral variations are seen during most of these nights such that the optical spectrum becomes bluer when brighter. The amplitude of variability is usually greater for longer observations but is lower when BL Lac is brighter. No evidence for periodicities or characteristic variability time-scales in the light curves was found. The color variations are mildly chromatic on long timescales.

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arXiv2017

Intra-night variability of the blazar CTA 102 during its 2012 and 2016 giant outbursts

We obtained and analyzed more than 100 hours of multicolour optical time series of the blazar CTA 102 during its 2012 and 2016 outbursts. The object reached almost 11-th mag at the end of 2016, which is perhaps the brightest blazar state ever observed! During both outbursts, CTA 102 showed significant and rapid variability on intra-night time scales, reaching up to 0.2 mag for 30 min on some occasions. The "rms-flux" relation, built for all datasets, shows a large scatter and no apparent saturation on the magnitude scale. The ensemble structure function of the light curves can be fitted well with a straight line of a slope of $\sim$0.4. The time lags between the different optical bands appear to be consistent with zero, taking into account our time resolution. We discuss different variability scenarios and favor the changing Doppler factor of the emitting blobs as the most plausible one to account for the observed intra-night variability.

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arXiv2017

Observations and light curve solutions of the eclipsing stars CSS J075205.6+381909 and NSVS 691550

The paper presents observations and light curve solutions of the eclipsing stars CSS J075205.6+381909 and NSVS 691550. As a result their initial epochs were determined. The target periods turned out almost equal to the previous values. We found that NSVS 691550 is overcontact system whose components are close in temperature while CSS J075205.6+381909 has almost contact configuration and temperature difference of its components is around 2000 K. Both targets undergo partial eclipses. Their stellar components seem to obey the relations mass-temperature of MS stars.

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arXiv2016

The cataclysmic variable AE Aquarii: B-V colour of the flares

We report simultaneous observations of the flaring behaviour of the cataclysmic variable star AE Aqr. The observations are in Johnson B and V bands. The colour-magnitude diagrams (B-V versus V and B-V vs. B) show that the star becomes blues as it becomes brighter. In our model AE Aqr behaviour can be explained with flares (fireballs) with 0.03 < B-V < 0.30 and temperature in the interval 8000 < T < 12000.

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arXiv2016

Optical light curves of FUor and FUor-like objects

Using recent data from photometric monitoring and data from the photographic plate archives we aim to study, the long-term photometric behavior of FUors. The construction of the historical light curves of FUors could be very important for determining the beginning of the outburst, the time to reach the maximum light, the rate of increase and decrease in brightness, the pre-outburst variability of the star. Our CCD photometric observations were performed with the telescopes of the Rozhen (Bulgaria) and Skinakas (Crete, Greece) observatories. Most suitable for long-term photometric study are the plate archives of the big Schmidt telescopes, as the telescopes at Kiso Observatory, Asiago Observatory, Palomar Observatory and others. In comparing our results with light curves of the well-studied FUors, we conclude that every new FUor object shows different photometric behavior. Each known FUor has a different rate of increase and decrease in brightness and a different light curve shape.

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arXiv2016

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

After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and gamma-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray emission regions. The relation between the gamma-ray and optical fluxes is consistent with the SSC mechanism, with a quadratic dependence of the SSC gamma-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the gamma-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in gamma-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.

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