arXiv subjects

S. G. Zheltoukhov

Publications and source records attributed to S. G. Zheltoukhov.

At least 19 recordsLinked to original sources

arXiv2026

Sky background accounting in spectral infrared observations of extended objects at the Caucasus Mountain Observatory of the SAI MSU

The Caucasus Mountain Observatory of the Sternberg Astronomical Institute of Moscow State University is the only one in Russia and one of the few in the world where is possible to obtain spectral data in the near-infrared (IR) range at $λ$=1-2.5 $μ$m. However, there is a problem of processing the spectra of extended objects, the angular dimensions of which exceed the length of the slit (4.5 arcmin). Obtaining additional spectra of the sky in the immediate vicinity of such objects does not solve the problem, since bright atmospheric hydroxyl lines at $λ$~2 $μ$m change their intensity significantly over a time shorter than the exposure time of a single frame. We have developed a technique that allows us to correctly account for and exclude the contribution of variable atmospheric lines in the spectra of extended objects. This technique has been successfully tested in spectroscopic studies of the star-forming region NGC 7538 (S158) in our Galaxy.

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arXiv2026

Anomaly detection in Fink. I. Discovery, follow-up, and classification of unusual sources

Modern wide-field time-domain surveys produce alert streams whose scientific potential is often concentrated in rare and unusual events. Efficient discovery therefore requires automated pipelines to be combined with rapid expert validation and follow-up. We present the first-year performance of the anomaly-detection (AD) pipeline operating within the Fink broker on the Zwicky Transient Facility alert stream, and assess its ability to identify scientifically valid outliers and enable discovery of rare phenomena. The pipeline transforms ZTF light curves into a compact set of features and ranks alerts using an Isolation Forest model trained on archival ZTF data. Each night, the 10 most anomalous candidates are distributed to experts via Slack/Telegram and exposed through an API. We also implement an expert-feedback loop using a public Telegram bot and retrain the model using the Active Anomaly Discovery algorithm. During the first year of operations (starting from 25 January 2023), the AD pipeline identified multiple high-interest sources and triggered dedicated photometric and spectroscopic follow-up. We report the discovery and multi-instrument (11-m SALT telescope, 2.5-m CMO telescope, 0.6-m ASA RC600, 0.25-m FRAM-ORM) follow-up of the rare AM CVn system Fink J062452.88+020818.3 of the WZ Sge type, UX Ori-type star Fink J222324.32+744222.0 and the unusual transient with precursor SN 2023mtp. In addition, the module triggered 33 supernovae, including 30 previously unreported ones, with candidates for superluminous and hostless events. Furthermore, nine new dwarf novae were discovered. These results show that broker-level anomaly detection, coupled with rapid dissemination, expert assessment, and follow-up observations, provide an effective bridge between large-scale survey streams and domain expertise, turning anomaly scores into astrophysical insights and concrete discoveries.

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arXiv2026

The eruptive young star IRAS 21204+4913

The results of photometric, polarimetric, and spectroscopic observations are presented for the young star IRAS 21204+4913, whose visible brightness has increased by $\approx 5^{\rm m}$ since October 2025. The star's absorption spectrum in the $0.36 - 0.75 μ$m range resembles those of A - F giants and supergiants, but it also exhibits molecular TiO bands. The brightening was accompanied by a significant increase in the degree of polarization of the stellar radiation (to $\approx 16 \%$ in the I-band), likely due to scattering by dust in an expanding circumstellar shell. The P Cygni profile of the H$α$ line implies a dusty wind velocity of $\approx 300$ km/s. We believe that the outburst of IRAS 21204+4913 is caused by an increase in the accretion rate of protoplanetary disk's matter onto the young star with a mass of $\lesssim 0.5$ M$_\odot$ to $\gtrsim 3\times 10^{-5}$ M$_\odot$ yr$^{-1}$. Furthermore, IRAS 21204+4913 displays several unusual features: the dependence of the width and radial velocity of absorption lines on the excitation potential, emission in the TiO molecular bands, and a comparably bright outburst that occured in 1948. Several T Tauri stars and a group of Herbig-Haro objects are found in the vicinity of the star.

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arXiv2024

Validation of up to seven TESS planet candidates through multi-colour transit photometry using MuSCAT2 data

The TESS mission searches for transiting exoplanets by monitoring the brightness of hundreds of thousands of stars across the entire sky. M-type planet hosts are ideal targets for this mission due to their smaller size and cooler temperatures, which makes it easier to detect smaller planets near or within their habitable zones. Additionally, M~dwarfs have a smaller contrast ratio between the planet and the star, making it easier to measure the planet's properties accurately. Here, we report the validation analysis of 13 TESS exoplanet candidates orbiting around M dwarfs. We studied the nature of these candidates through a multi-colour transit photometry transit analysis using several ground-based instruments (MuSCAT2, MuSCAT3, and LCO-SINISTRO), high-spatial resolution observations, and TESS light curves. We present the validation of five new planetary systems: TOI-1883b, TOI-2274b, TOI2768b, TOI-4438b, and TOI-5319b, along with compelling evidence of a planetary nature for TOIs 2781b and 5486b. We also present an empirical definition for the Neptune desert boundaries. The remaining six systems could not be validated due to large true radius values overlapping with the brown dwarf regime or, alternatively, the presence of chromaticity in the MuSCAT2 light curves.

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arXiv2024

Spectral energy distribution of late stage stars

This paper presents a catalog of the energy distribution in the spectra of 263 stars in the wavelength range from 0.4 to 100 μm, which are at late stages of evolution and have been observed by the ISO space observatory. For each object in the catalog, estimates of the observed bolometric fluxes were derived from smoothed energy distribution curves. The catalog is available at https://infra.sai.msu.ru/sai\_lss\_sed both as a table and in machine-readable format. It is shown that for the specified sample of objects their ISO SWS spectra in the range 2.4-45 μm only 60% of cases correspond to the general shape of the continuum, and can be used without recalibration. A selection of carbon stars, accessible for the infrared observations from the MSU observatories has been made. For some of them the first brightness estimates in the K, L, and M bands were obtained with the new IR camera of the 2.5-m telescope of CMO.

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arXiv2024

New infrared camera of the Caucasian Mountain Observatory of the SAI MSU: design, main parameters, and first light

This paper presents a prototype of an infrared photometer, created at SAI of MSU based on the commercial infrared module Gavin-615A. The operating spectral range of the photometer is 3-5 mkm. Investigations of the photometer's detector have shown that its parameters coincide with those stated by the manufacturer. The nonlinearity of the detector does not exceed approx 5% across the entire signal range, and coefficients for correction functions were determined. Additionally, we determined the readout noise RN=1200+-210 e-, the conversion coefficient GAIN=520+-9 e-/ADU, the signal magnitude of the bias frame BIAS = 960.5+-2.2 ADU, and the dark current 9.3e6 +-1.1e6 e-/s, which is the sum of the detector's dark current and the radiation from the entrance window of the detector module. The value of dark current was measured at a window temperature of 6C. Observations commenced with the photometer at the 2.5-m telescope of the Caucasian Mountain Observatory of MSU, with the first results presented in this paper. The unvignetted field of view was 30". In the M band under good atmospheric conditions, an image quality close to the diffraction limit was achieved. Images of a star with brightness L=7.96 and M=6.78 were obtained over the 20-second exposure time and a SNR=10. It is shown that at high image quality with SNR=3 and exposure of 20 seconds it is possible to observe stars up to L=9 mag and M=8 mag. The main module of the photometer was also used in measurements of the sky background brightness.

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arXiv2024

Post-AGB candidate IRAS 02143+5852: Cepheid-like variability, three-layer circumstellar dust envelope and spectral features

We present the results of multicolour $UBVR_{\text{C}}I_{\text{C}}JHK$ photometry, spectroscopic analysis and spectral energy distribution (SED) modelling for the post-AGB candidate IRAS 02143+5852. We detected Cepheid-like light variations with the full peak-to-peak amplitude $ΔV\sim0.9$ mag and the pulsation period of about 24.9 d. The phased light curves appeared typical for the W Vir Cepheids. The period-luminosity relation for the Type II Cepheids yielded the luminosity $\log L/L_{\odot}\sim2.95$. From a low-resolution spectrum, obtained at maximum brightness, the following atmospheric parameters were determined: $T_\text{eff}\sim7400$ K and $\log g\sim1.38$. This spectrum contains the emission lines H$α$, BaII $λ$6496.9, HeI $λ$10830 and Pa$β$. Spectral monitoring performed in 2019-2021 showed a significant change in the H$α$ profile and appearance of CH and CN molecular bands with pulsation phase. The metal lines are weak. Unlike typical W Vir variables, the star shows a strong excess of infrared radiation associated with the presence of a heavy dust envelope around the star. We modelled the SED using our photometry and archival data from different catalogues and determined the parameters of the circumstellar dust envelope. We conclude that IRAS~02143+5852 is a low-luminosity analogue of dusty RV Tau stars.

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arXiv2024

On the Effectiveness of Observations in the Mid-Infrared Wavelength Range on the 2.5-Meter Telescope of the Caucasus Mountain Observatory of Moscow State University with Commercial IR Cameras

The main factors that influence the success of observations in the infrared range (central wavelengths of the photometric bands at 3.75 and 4.8~$μ$m) on the multipurpose optical telescope are considered. Estimates of the sky background brightness are obtained for the Caucasus Mountain Observatory (CMO) of Moscow State University: $1.3\cdot10^6$~photons/(s pixel) in the 3.75~$μ$m band and $3.4\cdot10^6$~photons/(s pixel) in the 4.8~$μ$m; and the instrumental background for the 2.5-m CMO telescope at $0^\circ$C: $3.2\cdot10^6$~photons/(s pixel) in the 3.75~$μ$m band and $4.3\cdot10^6$~photons/(s pixel) in the 4.8~$μ$m band. It is shown that at this background signal level with the currently available commercial cameras in the $3-5$~$μ$m spectral range, the telescope-camera coupling capabilities for observing faint objects will still be limited by the thermal background. For different observational conditions, estimates of the limiting magnitudes of objects available for observations in the 3.75 and 4.8~$μ$m ranges are obtained. For average observation conditions (instrument temperature of $0^\circ$C and stellar image size of 1 arcsec), the limit is $\sim10.6^m$ and $\sim8.4^m$, respectively.

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arXiv2024

ZZ Tau IRS: a low mass UX Ori type star with strong wind

The results of photometric, polarimetric and spectroscopic observations of the young star ZZ Tau IRS in the visible and near-infrared bands are presented. Against the continuum of an M spectral type star about 50 emission lines of allowed (HI, HeI, NaI, SII) and forbidden (OI, OII, OIII, NI, NII, SII, CaII, FeII, NiII) transitions were identified. It was found that from the autumn of 2020 to the beginning of 2023, the brightness of the star in the visible region decreased $(ΔI \approx 1.5^m),$ and then began to return to initial level. As the visible brightness of the star declined, its colour indices decreased in the visible region, but increased in the near-IR bands. At light minimum, the degree of polarization in the $I$ band reached $\approx$ 13%, and the equivalent widths of e.g. the H$α$ and [SII] 6731 A lines increased to 376 and 79 A, respectively. Arguments are given in favour of ZZ Tau IRS being a UX Ori type star, and its variability being due to eclipses by dust clouds, which are inhomogeneities in the dusty disc wind. Forbidden lines are formed both in the disc wind and in the jet, the axis of which is oriented along PA$=61\pm 3$ degrees. The jet mass-loss rate is $>5 \times 10^{-10}$ M$_\odot$/yr, what is abnormally large for a star with a mass of $<0.3$ M$_\odot.$ Apparently, the disc wind of ZZ Tau IRS is not axially symmetric, probably due to the azimuthal asymmetry of the protoplanetary disc found earlier from ALMA observations.

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arXiv2023

Recurrent Symbiotic Nova T Coronae Borealis Before Outburst

The results of photometric and spectral observations of T CrB obtained in a wide range of wavelengths in 2011-2023 are presented. We use the near-IR light curves to determine a new ephemeris $JD_{min} = 2455828.9 + 227.55 \times E$ for the times of light minima when the red giant is located between the observer and the hot component. The flux ratio H$α$/H$β$ varied from $\sim 3$ to $\sim 8$ in 2020-2023, which may be due to a change in the flux ratio between the X-ray and optical ranges. It is shown that the value of H$α$/H$β$ anticorrelates with the rate of accretion onto the hot component of the system. Based on high-speed follow-up observations obtained on June 8, 2023, we detected a variability of the HeII $λ4686$ line with a characteristic time-scale of $\sim 25$ min, the amplitude of variability in the $B$-band was $\sim 0.07^m$. Simulations of the near-IR light curves accounting for the ellipsoidal effect allowed us to obtain the parameters of the binary system: the Roche lobe filling factor of the cool component $μ=1.0$, the mass ratio $q=M_{cool}/M_{hot} \in [0.5, 0.77]$, the orbital inclination $i \in [55^\circ, 63^\circ]$. A comparison of the light curve obtained in 2005-2023 with the 1946 outburst template made it possible to predict the date of the upcoming outburst - January 2024.

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arXiv2023

Gas Kinematics in the Magellanic-Type Galaxy NGC 7292

The paper presents results of studying the kinematics of the ionized gas in the galaxy of the Large Magellanic Cloud type NGC 7292 obtained with the 2.5-m telescope of the Caucasian Mountain Observatory (CMO SAI MSU) and the 6-m BTA telescope of the Special Astrophysical Observatory (SAO RAS). Analysis of the velocity fields of the ionized and neutral hydrogen showed that the kinematic center of NGC 7292 located at the center of the bar, northwest of the photometric center of the galaxy (the southeastern end of the bar) previously taken as the center of NGC 7292. In addition to the circular rotation of the gas, the radial motions associated with the bar play a significant role in the kinematics of the disk. The observed perturbations of the gaseous-disk kinematics induced by the ongoing star formation do not exceed those caused by the bar. It is possible that part of the non-circular motions (at the southeastern end of the bar which is the brightest HII region) may be related to the effects of the capture of a dwarf companion or a gaseous cloud.

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arXiv2023

The Post-AGB Star IRAS 07253-2001: Pulsations, Long-Term Brightness Variability and Spectral Peculiarities

The observations and comprehensive study of intermediate initial mass stars at the late stages of evolution, and after the asymptotic giant branch (AGB) in particular, are of crucial importance to identify the common properties for the stars of given group and to reveal binaries among them. This work aims to investigate photometric and spectral peculiarities of a poorly studied post-AGB candidate and infrared source IRAS 07253-2001. We present the new multicolour $UBVR_{C}I_{C}YJHK$ photometry obtained with the telescopes of the Caucasian mountain observatory and analyse it together with the data acquired by the All Sky Automated Survey for SuperNovae. We report on the detection of multiperiod brightness variability caused by pulsations. A beating of close periods, the main one of 73 days and additional ones of 68 and 70 days, leads to amplitude variations. We have also detected a long-term sine trend in brightness with a period of nearly 1800 days. We suppose it to be orbital and IRAS 07253-2001 to be binary. Based on new low-resolution spectroscopic data obtained with the 2.5-m telescope of the Caucasian mountain observatory in 2020 and 2023 in the $λ$3500-7500 wavelength range we have identified spectral lines and compiled a spectral atlas. We have found the [N II], [Ni II] and [S II] forbidden emission lines in the spectrum and discuss their origin. The H$α$ line has a variable double-peaked emission component. We have derived preliminary estimates of the star's parameters and detected a variation of radial velocity with a peak-to-peak amplitude of about 30 km s$^{-1}$.

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arXiv2022

SRGe2149+6736 -- the new candidate to AM~Her type variables discovered by eROSITA telescope on "Spectrum--Roentgen--Gamma" orbital observatory

We present the results of the optical identification, classification, as well as analysis of photometric and spectral observations of the X-ray transient SRGe2149+6736 detected by the eROSITA telescope during SRG all-sky X-ray survey. Photometric observations of the optical companion of SRGe2149+6736 were carried out on 6m telescope BTA SAO RAS, 1.5m Russian-Turkish telescope RTT-150 and 2.5m telescope CMO of Moscow State University. Together with ZTF data they showed that the source is a cataclysmic variable with an orbital period $P=85\pm0.4$~min which demonstrates long-term brightness variability from $23.5$~mag (low state) to $20$~mag (high state). The high-state light curves are consistent with a model of accreting magnetic white dwarf and suggest that SRGe2149+6736 belongs to AM~Her type variables. The optical spectra obtained in the low state are consistent with a spectral energy distribution of a white dwarf with a temperature of ~24000 K.

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arXiv2022

Observations of the luminous red nova AT 2021biy in the nearby galaxy NGC 4631

We present an observational study of the luminous red nova (LRN) AT\,2021biy in the nearby galaxy NGC\,4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from $\sim 231$\,days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a bolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed by the longest plateau among LRNe to date, with a duration of 210\,days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT\,2021biy has local dust properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a $\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) = 5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17--24\,\msun\ primary component.

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arXiv2021

Peculiar X-ray transient SRGA J043520.9+552226/AT2019wey discovered with SRG/ART-XC

Context: During the ongoing all-sky survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory should discover new X-ray sources, many of which can be transient. Here we report on the discovery and multiwavelength follow-up of a peculiar X-ray source SRGA J043520.9+552226=SRGe J043523.3+552234 - the high-energy counterpart of the optical transient AT2019wey. Aims: Thanks to its sensitivity and the survey strategy, the Mikhail Pavlinsky ART-XC telescope uncovers poorly studied weak transient populations. Using a synergy with current public optical surveys, we are aiming at revealing the nature of these transients to study its parent populations. The SRGA J043520.9+552226 is the first transient detected by ART-XC which has a bright optical counterpart suitable for further studies. Methods: We have used available public X-ray and optical data and observations with SRG, INTEGRAL, NuSTAR, NICER and ground-based telescopes to investigate the source spectral energy distributions at different phases of the outburst. Results: Based on X-ray spectral and timing properties derived from space observations, optical spectroscopy and photometry obtained with the 2.5-m and RC600 CMO SAI MSU telescopes, we propose the source to be a black hole in a low-mass close X-ray binary system.

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arXiv2021

Anomaly detection in the Zwicky Transient Facility DR3

We present results from applying the SNAD anomaly detection pipeline to the third public data release of the Zwicky Transient Facility (ZTF DR3). The pipeline is composed of 3 stages: feature extraction, search of outliers with machine learning algorithms and anomaly identification with followup by human experts. Our analysis concentrates in three ZTF fields, comprising more than 2.25 million objects. A set of 4 automatic learning algorithms was used to identify 277 outliers, which were subsequently scrutinised by an expert. From these, 188 (68%) were found to be bogus light curves -- including effects from the image subtraction pipeline as well as overlapping between a star and a known asteroid, 66 (24%) were previously reported sources whereas 23 (8%) correspond to non-catalogued objects, with the two latter cases of potential scientific interest (e. g. 1 spectroscopically confirmed RS Canum Venaticorum star, 4 supernovae candidates, 1 red dwarf flare). Moreover, using results from the expert analysis, we were able to identify a simple bi-dimensional relation which can be used to aid filtering potentially bogus light curves in future studies. We provide a complete list of objects with potential scientific application so they can be further scrutinised by the community. These results confirm the importance of combining automatic machine learning algorithms with domain knowledge in the construction of recommendation systems for astronomy. Our code is publicly available at https://github.com/snad-space/zwad

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arXiv2020

Transient Double-beam Spectrograph for the 2.5-m Telescope of the Caucasus Mountain Observatory of SAI MSU

The Transient Double-beam Spectrograph (TDS) is designed for optical low-resolution observations of non-stationary and extragalactic sources with the 2.5-m telescope of Caucasus Mountain Observatory of the Sternberg Astronomical Institute. It operates simultaneously in a short-wavelength (360--577 nm, reciprocal dispersion 1.21 A/pixel, resolving power R=1300 with a 1 arcsec slit) and long-wavelength (567--746 nm, 0.87 A/pixel, R=2500) channels. The light is split by a dichroic mirror with a 50% transmission at 574 nm. In the "blue" channel, the automatic replacement of the grating by a grism with a double resolving power is possible. Two CCD-cameras use E2V 42-10 detectors cooled down to $-70^\circ$C with a readout noise of 3 $e-$ at a readout rate of 50 kHz. The spectrograph is equipped with a back slit viewer camera and a calibration unit allowing to record a comparison spectrum from a hollow cathode lamp for wavelength calibration or from an LED source with a continuous spectrum (the "flat field") to take into account the vignetting and uneven slit illumination. The throughput of the entire optical path without slit loss is 20% at the zenith in the "blue" and 35% in the "red" channel. Excluding the atmosphere and the telescope, the efficiency of the TDS itself reaches a maximum of 47% and 65% respectively. The spectrograph is permanently mounted in the Cassegrain focus of the 2.5-m telescope of CMO SAI MSU sharing the port with a wide-field photometric CCD-camera. The spectrograph is fed by the light from a folding mirror introduced into the optical path. Since November 2019, TDS has been used for regular observations of non-stationary stars and extragalactic sources up to 20-th mag in a 2-h exposure with a signal-to-noise ratio >5 per pixel.

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arXiv2020

Discovery of a hot ultramassive rapidly rotating DBA White Dwarf

We report the discovery of a nearby massive white dwarf with He-H atmosphere. The white dwarf is located at a distance of $74.5\pm0.9$ pc. Its radius, mass, effective temperature, H/He ratio and age are $R=2500\pm100$ km, $M=1.33\pm0.01$ $\rm M_{\odot}$, $T_{\mathrm{eff}}=31200\pm 1200$ K, ${\mathrm{H/He}}\sim0.1$ and $330\pm40$ Myr, respectively. The observed spectrum is redshifted by $V_{\mathrm{r}}=+240\pm15$ km s$^{-1},$ which is mostly attributed to the gravitational redshift. The white dwarf shows a regular stable photometric variability with amplitude $Δg\approx0.06^{\mathrm{m}}$ and period $P=353.456$ s suggesting rapid rotation. This massive, hot and rapidly rotating white dwarf is likely to originate from the merging of close binary white dwarf system that avoided explosion in a thermonuclear type Ia supernova at the Carboniferous Period of the Earth history.

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