arXiv subjects

A. V. Dodin

Publications and source records attributed to A. V. Dodin.

At least 19 recordsLinked to original sources

arXiv2026

In the Eye of the Storm: The Third Giant X-ray Outburst of the Extreme Changing-look AGN IC 3599

We report the discovery and multiwavelength follow-up observations of a giant (factor >100) X-ray outburst of the exceptional changing-look active galactic nucleus (AGN) IC 3599. This is the third such outburst after two previous ones serendipitously discovered in 1990 and 2010. Based on our dedicated long-term monitoring of IC 3599 with {\it Swift}, the third outburst was detected while it was happening, and we triggered multiple follow-up observations within days to weeks for the first time. The Swift outburst spectra are supersoft and almost no photons are detected beyond 2.5 keV. The XMM-Newton short-term light curve shows a remarkable apparent oscillatory pattern that is reminiscent of quasiperiodic oscillations (QPOs). The optical high-state spectra reveal a multitude of bright coronal emission lines that have dramatically brightened and were absent or much fainter in low-state spectra. The new results eliminate outburst scenarios that require a constant time interval of repetitions (like certain variants of repeat tidal stripping, or of an orbiting supermassive black hole impacting the inner accretion disk), but remain in excellent agreement with an accretion disk radiation-pressure instability when assuming that local conditions in the disk of this long-lived AGN affect the onset time of each new instability. The combination of recurrent, giant, supersoft outbursts on decadal timescales, the exceptional emission-line response, and the rapid, candidate quasiperiodic, short-term variability on an hours timescale makes IC 3599 unique among AGN, and establishes it as a key system for studying accretion physics under extreme conditions and at the Eddington limit.

astro-ph.GA↗

arXiv2026

Photometric and spectral variability of the candidate post-AGB star IRAS 07506-0345

We report photometric and spectroscopic observations of the poorly studied post-AGB candidate IRAS 07506-0345, obtained with the telescopes of the Caucasian Mountain Observatory of SAI MSU in 2023-2026. The star exhibits Cepheid-like variability with a pulsation period of P = 33.68 days. Low-resolution spectra reveal a strong CN molecular band at $λ$ 3883, characteristic of carbon-rich post-AGB objects, while H$α$ shows phase-dependent emission and P~Cyg-type profiles, indicating shock waves and mass outflow. Radial velocities are measured from multiple spectral lines. Using period-luminosity-color relations for Type II Cepheids, we estimate a luminosity of $L = 1050\pm250\,L_\odot$. The combined properties - period, amplitude, light-curve morphology, infrared excess, low metallicity, and hydrogen line emissions - securely classify IRAS 07506-0345 as an RV Tau variable.

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arXiv2026

The curved jet in the young star FN Tau

In the vicinity of the young star FN Tau, we have detected a microjet and four Herbig-Haro objects, whose positions and kinematics indicate the presence of a bipolar collimated outflow from the star - HH 1267. The stellar jet does not propagate rectilinearly, and we discuss the possibility that the curved shape of the jet, whose axis is inclined to the line of sight at an angle $<20^\circ$, results from the precession of the inner regions of the FN Tau protoplanetary disk. Approximately 60 years ago, the star underwent outbursts with an amplitude of $Δm_{\rm pg} \sim 2^{\rm m}$ lasting several months, which we associate with the onset of the microjet.

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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.

astro-ph.HE↗

arXiv2026

Photometric and Spectral Evolution of the Symbiotic Nova HM Sagittae since 2003

We present photometric and spectral monitoring of the symbiotic system HM Sge, which consists of a Mira variable with a dust shell and a hot white dwarf ionizing the surrounding gaseous nebula. The system underwent a nova-like outburst in 1975 and experienced a high-activity episode during 2018-2021. UBV photometry from 2003 to 2025 shows monotonic fading at about 0.05 mag/yr until 2018, followed by a 0.3 mag brightening peaking in 2021 and a decline by 2025 to the faintest level in five decades of monitoring. Near-infrared observations (JHKLM, 2009-2025), combined with archival data, reveal Mira pulsations with a period of 532 d and long-term variability driven by changes in the optical depth of the dust shell. Spectral monitoring (2016-2025) reveals a substantial evolution in the emission spectrum of the gaseous envelope. The 2018-2021 high-activity episode was accompanied by enhanced fluxes in recombination lines (H I, He I, He II) and forbidden transitions ([O I], [Ar V], [Fe VI], [Ca VII]), together with a 17-fold brightening of the Raman-scattered O VI $λ$6725 line. We report the first detection of the [Fe X] $λ$6374 line in HM Sge. Monitoring this line from 2007 to 2025 shows its equivalent width growing through 2017 - indicating gradual coronal heating - then declining by 2021, likely reflecting altered accretion conditions and/or hot-component properties during the high-activity episode. We propose that both the 1975 outburst and the 2018-2021 high-activity episode may be linked to periastron passage of the binary components; if so, the about 46-year interval would constrain the system's orbital period.

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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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arXiv2026

The radial velocity curve for HeII emission cannot be used for component mass determination in SS433

More than 150 measurements of the HeII 4686A emission line in spectra of SS433 were obtained during 388 nights in 2020-2025 with the Transient Double-beam Spectrograph on the 2.5 m telescope of Caucasian Mountain Observatory of Sternberg Astronomical Institute. We found that the HeII emission line formation region is not eclipsed and is significantly larger than both the donor star and the photosphere of the supercritical accretion disk. The HeII radial velocity curve was found to be independent of the precessional phase and inconsistent with the photometric curve. These findings suggest that the HeII line does not reflect the orbital motion of the compact object. Therefore, spectroscopic estimates of the masses of the components in SS433 based on the HeII emission line can be unrealistic.

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arXiv2025

On the influence of component orbital motion on the photometric variability of DF Tau

Based on the analysis of the long-term light curve of the young binary DF Tau spanning approximately 125 years, we infer that its brightness variations are associated with changes in the accretion rate from the circumstellar protoplanetary disk onto the primary. We have also substantially improved the orbital parameters of DF Tau, which enables us to align its secular light curve with the evolution of the binary's component separation. The relationship between the long-term brightness variations and the orbital motion of DF Tau, if present, appears to be inconsistent with theoretical predictions. Notably, similar discrepancies between theory and observations are also seen in other young binary systems. Furthermore, the source of the polarized radiation in the optical range is found to be located at a distance of $\lesssim 0.5^{\prime\prime}$ from the star, with the polarization variability showing no dependence on the orbital phase.

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arXiv2025

Unique microquasar SS433: new results, new issues

The unique microquasar SS433 is a massive X-ray binary system at an advanced evolutionary stage. The optical star overflows its Roche lobe and transfers mass at a very high rate onto a black hole surrounded by a supercritical accretion disk with relativistic jets inclined to the orbital plane. Both disk and jets precess with a period of 162.3 days. In the outer parts of the precessing jets, emission lines of hydrogen and neutral helium are formed, moving periodically across the spectrum of SS433 with an enormous amplitude of $\sim 1000$\,Å or, on the $\sim 50000$ km/s velocity scale. A 30-year spectral and photometric monitoring of SS433 has been carried out at Sternberg Astronomical Institute. Using all published data for 45 years of observations, we obtained a number of important results concerning the nature of this unique microquasar. We discovered a secular evolutionary increase in the orbital period of SS433 at a rate of $(1.14 \pm 0.25)\times 10^{-7}$ seconds per second, suggesting that the relativistic object in SS433 is a black hole with mass exceeding 8\,${\rm M}_\odot$. It is shown that the distance between the components of SS433 increases with time, which prevents the formation of a common envelope in the system. The size of the Roche lobe of the optical donor star is on average constant in time, which ensures a stable secondary mass exchange in the system. The orbital ellipticity of SS433 was discovered, strongly supporting the model of a slaved accretion disk tracking the precession of the rotation axis of the optical star, which is inclined to the orbital plane due to an asymmetric supernova explosion. Parameters of the kinematic model of SS433, except for the precession period, keep on average constant for 45 years. Phase jumps of the precession period were detected, but on average the precession period remains constant for 45 years. (Abridged)

astro-ph.HE↗

arXiv2025

The jet and circumstellar environment of the young binary DF Tau

Jets and disc winds play an important role in the evolution of protoplanetary discs and the formation of planetary systems. However, there is still a lack of observational data regarding the presence and parameters of outflows, especially for close young binaries. In this study, we aim to find the HH flow near the young sub-arcsecond binary DF Tau and explore its morphology. Narrow-band H$α$ and H$_2$ 2.12 $μ$m imaging and spectroscopic observations of DF Tau and its vicinity were performed. We have discovered several emission nebulae near the binary, which likely result from the interaction of gas outflow from the binary components with the surrounding medium. The outflow appears to occur both in the form of jets, generating numerous Herbig-Haro objects (HH 1266 flow), and as a weakly collimated wind responsible for the formation of the ring-like nebula around the binary and the rim of the cometary globule. We have found that the angle between the jet and the counter-jet is $168^\circ$ and discuss the complex morphology of the HH flow.

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arXiv2025

On the causes of brightness variability of the young star BP Tau

We have constructed and analysed the secular light curve of BP Tau, a classical T Tauri-type star. Wave-like variations in the average brightness were detected, with an amplitude of $ΔB\approx 0.2$ and characteristic time-scales of several decades. We argue that three deep dimming events $(ΔB \sim 1.5)$, lasting from 1 hour to several days, are caused by the eclipse of a hot (accretion) spot by dust falling onto the star together with gas. Such eclipses, albeit with smaller amplitudes, may explain the absence of a strictly defined periodicity in the brightness variations of BP Tau associated with axial rotation. We also show that within the distance range of 0.1 to 200 AU, BP Tau does not have a companion with a mass exceeding 0.2M$_\odot.$ The causes of brightness and colour index variations on different time-scales are discussed.

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arXiv2025

Evidence for symbiotic nature of 2MASS J21012803+4555377

Just less than 300 symbiotic stars are currently known in the Galaxy. The population synthesis methods predict that this amount should be 10--100 times larger. In recent years, several works have attempted to find symbiotic candidates from photometric surveys. Regular spectroscopic observations of these candidates can increase the number of known symbiotic systems. We aim to verify the symbiotic nature of 2MASS J21012803+4555377. Archive photometry and low-resolution spectra (R=1500-2000) of the candidate are presented. Besides molecular bands, the spectrum displays numerous emission lines including forbidden and high-ionization ones -- He II, [Fe VII], etc. A strong Raman-scattered O VI line is observed at 6825A. The source shows prominent excessive emission in the infrared related to a dense circumstellar dust shell. All this evidence indicates that 2MASS J21012803+4555377 should be classified as a D-type symbiotic star.

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arXiv2025

Transient narrow high-velocity absorptions in the stationary spectra of SS433

We report on the discovery of rare emergence (31 nights from 360 nights of observations) of narrow absorption features in hydrogen and helium lines in stationary SS433 spectra with velocities ranging from $-650$ to $-1900$ km/s. The components arise independently of the appearance of P-Cygni line profiles which are frequently observed in the SS433 stationary spectra with terminal velocities ranging from $-200$ to $\sim -2500$ km/s. The characteristic rising time of the transient absorptions is about one day and the decay time is about two days. The phenomenology of the absorptions suggests their origin due to hydrodynamic instabilities of wind outflows from a supercritical accretion disk in SS433.

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arXiv2024

Radial velocities of narrow emission line components in the spectra of T Tauri stars

We studied rotational modulation of the radial velocities of narrow emission lines in four classical T Tauri stars. We found that the previously declared shift of the mean velocity of neutral and ionized helium lines relative to the mean radial velocity of the star is not associated with the inflow of accreted gas into the hotspot, since the radial velocity curves for lines with different velocity shifts should exhibit phase shifts relative to each other, while the observed phase shifts are absent within their uncertainties and do not correspond to the observed line velocity shifts. This means that the line shifts are not caused by the actual gas motion. For neutral helium lines, the shifts can be explained by the large optical thickness of the lines and the Stark effect at plasma parameters expected at the base of the accretion column of T Tauri stars.

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arXiv2024

The jet of BP Tau

A strong global magnetic field of young low-mass stars and a high accretion rate are the necessary conditions for the formation of collimated outflows (jets) from these objects. But it is still unclear whether these conditions are also sufficient. We aim to check whether BP Tau, an actively accreting young star with a strong magnetic field, has a jet. We carried out narrowband SII 672 nm imaging and spectroscopic observations of BP Tau and its vicinity. We find that BP Tau is a source of a Herbig-Haro flow (assigned number HH 1181), which includes two HH objects moving from the star in opposite directions and a micro- (counter-) jet of ~ 1" projected length. The flow is oriented along position angle $59 \pm 1$ degree.

astro-ph.SR↗

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

Optical Identification and Spectroscopic Redshift Measurements of 216 Galaxy Clusters from the SRG/eROSITA All-Sky Survey

We present the results of the optical identification and spectroscopic redshift measurements of 216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observations were performed in 2020-2023 with the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope at the Sayan Solar Observatory of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, and the 1.5-m Russian-Turkish telescope (RTT-150) at the TÜBİTAK Observatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements have been obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in the SRG/eROSITA survey and 22 galaxy clusters are at redshifts $z_{spec} \gtrsim 0.7$, including three at $z_{spec} \gtrsim 1$. Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters at $z_{spec} > 0.7$. For these observations the most massive clusters are selected. Therefore, most of the galaxy clusters presented here most likely will be included in the cosmological samples of galaxy clusters from the SRG/eROSITA survey.

astro-ph.GA↗

arXiv2024

Spectroscopic Searches for Evolutionary Orbital Period Changes in WR+OB Binaries: the Case of WR 127 (Hen 3-1772)

We aim at searching for secular evolution of the orbital period in the short-period binary system WR 127 (WN3b+O9.5V, $P = 9.555^d$). We performed new low-resolution spectroscopic observations of WR 127 on 2.5-m CMO SAI telescope to construct the radial velocity curves of the components suggesting the component masses $M_\mathrm{WR}\sin^3(i) = 11.8\pm1.4$ $M_{\odot}$, $M_\mathrm{O}\sin^3(i)=17.2\pm1.4$ $M_{\odot}$. The comparison with archival radial velocity curves enabled us to calculate the $(O-C)$ plot with accuracy sufficient to search for the orbital period change in WR 127. We report on the reliable detection of a secular increase in the orbital period of WR 127 at a rate of $\dot{P} = 0.83\pm0.14~\mbox{s~yr}^{-1}$ corresponding to the dynamical mass-loss rate from the WR star $\dot{M}_\mathrm{WR} = (2.6\pm0.5)\times 10^{-5}$ $M_{\odot}$ yr$^{-1}$. The mass-loss rate from WR stars in three WR+OB binaries (WR 127, CX Cep and V444 Cyg) as inferred from spectroscopic and photometric measurements suggests a preliminary empirical correlation between the WR star mass and the dynamical mass-loss rate $\dot M_\mathrm{WR}\sim M_\mathrm{WR}^{1.8}$. This relation is important for the understanding of the evolution of massive close binaries with WR stars -- precursors of gravitational-wave binary merging events with neutron stars and black holes.

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