V1413 Aql is an anomalous classical symbiotic star that has remained in an active state almost continuously. After failing to return to quiescence following the 2020 outburst, it underwent a new flare in 2025. We present an analysis of new photometric and spectroscopic observations, which provide estimates for the binary components at different stages of activity: Lcool=1700L⊙, Rcool=130R⊙; Lhot=800L⊙, Rhot=28R⊙ during minimum; Lhot=7000L⊙, Rhot=64R⊙ at the 2025 maximum. The significant increase in the luminosity of the hot component during the 2025 outburst deviates from the typical behavior observed in classical symbiotic stars, where outbursts are generally attributed to a redistribution of spectral energy rather than intrinsic changes in luminosity.

Results of the simulation of the dependence of the ‘‘red leak’’ magnitude of photometric filters on various factors during star observations are presented: the colour index V – R , luminosity class, magnitude of interstellar reddening, air mass, and precipitable water vapour in the Earth’s atmosphere. The error arising from the neglect of ‘‘red leak’’ in the case of filters used on the 0.6-m telescope of the SAI CMO can be up to 0.6^m – 0.8^m for stars of late spectral types. Algorithms for the reduction of observational data for the U and B filters are presented. The results of observations of the rapid variability of two symbiotic stars CH Cyg and SU Lyn with cold components of very late spectral types are provided. For CH Cyg, rapid variability was detected on both observation dates. Taking into account the ‘‘red leak’’ effect, the amplitude of brightness changes in the B band was 0.10 ^m on November 6, 2019 and 0.19 ^m on December 15, 2022, with a characteristic variability time of about 20 min. For SU Lyn, rapid brightness variability in the B band on February 25, 2023 was not detected (with an accuracy of up to 0.003 ^m ).

The results of measurements of background brightness in the near-infrared range (J, H, K bands), carried out in 2016–2023 at the Caucasus Mountain Observatory of Moscow State University was analyzed. It is shown that the instrumental background associated with the thermal radiation of the telescope is noticeable only in the K band, and at operating temperatures its contribution mainly determines the level of the overall background in this band. The coefficients of a polynomial taking into account the contribution of instrumental and extra-atmospheric backgrounds are presented. It is shown that the brightness of the sky background does not depend on air temperature, but a weak dependence on the water vapor content is observed, close to that expected from model calculations: in the J and H bands, the background brightness decreases at a rate of ≈ 1pt 1%/1 mm, and in the K band it grows at a rate of ≈ 1pt 2.5%/1 mm. The maximum amplitude of background brightness variability on short time scales ( ∼ 1pt 30 min) has been estimated: ≈ 1pt 10 J and K bands and ≈ 1pt 30 H band. The maximum contribution of Moon’s radiation scattered in the atmosphere to the overall background level has been determined. It is shown that this contribution can be ignored at an angular distance from the Moon greater than ∼ 1pt 10^∘ even during a full moon. The average background surface brightness mag/arcsec2 in the J, H, and K bands was calculated: m_J = 15.7 , m_H = 13.9 , and m_K = 13.1 .

We present the results of a new X-ray to near-IR photometric and spectroscopic monitoring of the changing look active galactic nucleus NGC 2617 carried out from March 2022 to March 2024. We found significant variability at all wavelengths and in the intensities and profiles of the broad Balmer lines. Reverberation mapping was carried out for three observing seasons during 2021-2024. We obtained time delays of $\sim$4 d for the response of the H ${\beta }$ line to optical continuum variations. The X-ray variations correlate well with the UV and optical, with a few days lag for longer wavelengths. The K band lagged the B band by $\sim$ 15 d during the last three seasons, which is significantly shorter than the delays reported previously by the 2016 and 2017-2019 campaigns. Near-IR variability arises from two different emission regions: the outer part of the accretion disc and a more distant dust component. The $HK$-band variability is governed primarily by dust. The H ${\beta }$/H ${\alpha }$ ratio variations (for broad components only) correlate with the X-ray and UV fluxes. The spectral type changed from type 1.8 to type 1.9 during 2023 October-2024 January and then a more rapid change to type 1.5 occurred in 2024 February. We interpret these changes as a combination of two factors: changes in the accretion rate as a dominant cause but also the sublimation or recovery of dust along the line of sight.

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 (H I, He I, Na I, S II) and forbidden (O I, O II, O III, N I, N II, S II, Ca II, Fe II, Ni II) 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 (Δ I≈ 1m.5 ) and then began to return to the initial level. As the visible brightness of the star declined, its color 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 [S II] λ 6731 lines increased to 376 and 79 Å^∘ , respectively. Arguments are given in favor 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 disk wind. Forbidden lines are formed both in the disk wind and in the jet, the axis of what is oriented along PA=61^∘± 3^∘ . The jet mass-loss rate exceeds 5× 10^-10M_⊙ yr ^-1 , what is abnormally large for a star with a mass less than 0.3M_⊙ . Apparently, the disk wind of ZZ Tau IRS is not axially symmetric, probably due to the azimuthal asymmetry of the protoplanetary disk found earlier from ALMA observations.

The paper presents a prototype of an infrared photometer, made by the SAI of MSU on the base of a commercial infrared module Gavin-615A. The operating spectral range of the photometer is 3-5 µm. Studies of the photometer detector have shown that its parameters correspond to those declared by the manufacturer. Nonlinearity of the detector does not exceed ∼ 5% in the whole range of signals, coe cients of correction--function were determined. Additionally, we determined the readout noise RN = 1200 ± 210 e, conversioncoe cient GAIN = 520 ± 9 e/ADU, the BIAS = 960.5 ± 2.2 ADU, and dark current ≈ (9.3 ± 1.1) · 106-e /s,which consists of the sum of the detector dark current and radiation of the entrance window of the detector◦module. The value of dark current was measured at the window temperature of 6 C. Observations werestarted with the photometer at the 2.5-m telescope of the Causasian Mountain Observatory of MSU, the rst results are given in the paper. The unvignetted eld of view is 30'' . In the M band under good atmosphericconditions, an image quality is close to di raction limit. Images of a star with magnitude L = 7.96 and M = 6.78 were obtained with exposure of 20 seconds and a SNR∼10 ratio. 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 ∼9m and M ∼8m . The main module of the photometer was also used for measurements of the sky background brightness.

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$\alpha$/H$\beta$ 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$\alpha$/H$\beta$ 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 $\lambda 4686$ 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 $\mu=1.0$, 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.

Context. 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. Aims. We aim to check whether BP Tau, an actively accreting young star with a strong magnetic field, has a jet. Methods. We carried out narrowband [S II] 672 nm imaging and spectroscopic observations of BP Tau and its vicinity. Results. 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 ± 1°.

ABSTRACT We present the results of multicolour UBVRCICJHK 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 ∼ 0.9 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 logL/L⊙ ∼ 2.95. From a low-resolution spectrum, obtained at maximum brightness, the following atmospheric parameters were determined: Teff ∼ 7400 K and logg ∼ 1.38. This spectrum contains the emission lines H α, ${\rm Ba\, {\small II}}$ λ6496.9, ${\rm He\, {\small I}}$λ10830, 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.

We present the results of the infrared photometric observations in the JHKLM bands and infrared spectroscopic observations in the range 1–2.5 μ m for the carbon Mira star T Dra performed from 2019 to 2023. An analysis of the photometric observations shows the presence of both pulsational brightness fluctuations with an amplitude falling from 1.2^m in the J band to 0.84^m in the L and M bands and a linear trend in the mean brightness with a value of 0.0007^m/d in the J band. In the infrared spectrum of T Dra we have identified the absorption bands of C _2 H _2 , HCN, CN, CO, and C _2 molecules. The depth of the absorption band at 1.53 μ m has been found to depend on the star’s brightness. We show that the CO λ 2.29 μ m bands have a high contrast, suggesting their formation not in the stellar atmosphere but in the circumstellar dust envelope. We present the spectral energy distribution of T Dra in a wide spectral range from which the bolometric fluxes at maximum and minimum brightness have been estimated: 4.8× 10^-10 and 2.5× 10^-10 W m ^-2 , respectively. For the distance to T Dra of 944 pc they correspond to the star’s luminosity at maximum brightness L_max≈ 13 300 L_⊙ and at minimum brightness L_min≈ 6900 L_⊙ . We have modeled the radiative transfer in the circumstellar envelope of T Dra and estimated the parameters of the star and the envelope: T_eff=2400 K, R_*=670R_⊙ , R_in=5-6 AU, R_out∼ 50 000 AU, τ_V=3.5 , M_dust=4-8× 10^-5 M_⊙ , and dM/dt∼ 1.5× 10^-6 M_⊙ yr ^-1 .

This paper presents a catalog of the energy distribution in the spectra of 263 stars in the wavelength range 0.4 - 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 are 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 in 60% of cases correspond to the general shape of the continuum, and can be used without recalibration. A selection of carbon stars accessible to infrared observations from the MSU observatories was 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.

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≳ 0.7 , including three at z_spec≳ 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 we chose the most massive clusters and, therefore, most of the galaxy clusters presented here with the spectroscopic redshifts measured by us will most likely enter in future into the cosmological samples of galaxy clusters from the SRG/eROSITA survey.

The results of the analysis of photometric and spectral monitorings of the recurrent symbiotic nova T CrB, carried out using the low-resolution spectrograph TDS of the 2.5-m telescope and the CCD photometer of the 0.6-m telescope of the CMO SAI, are presented. It is shown that the fluxes in the emission lines H α , H β , He I λ 5876 change by 6–16 % , and in the emission line He II λ 4686 by about 60 % on timescales of 20–60 minutes. The line flux curves on August 25, 2020 are similar to the B light curve of T CrB, but with a time delay of up to 600 s for different lines. The observed spectrum is approximated by the radiation of the system components with the following parameters: a red giant of spectral type M4 III, a nebula with T_e=10^4 K and emission measure of 4× 10^58 cm ^-3 , and the accretion disk with R_1=0.003R_⊙ , R_out≈ 1R_⊙ (at inclination i=57^∘ ).

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 multicolor UBVR_CI_CYJHK 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 .

The morphology and kinematics of the matter in the vicinity of young stars ZZ Tau and ZZ Tau IRS are studied. It has been found that the emission nebula (the H α filament) located southwest of these stars, as well as the Herbig–Haro object HH 393 projected onto it are moving away from ZZ Tau and ZZ Tau IRS with a radial velocity of about 50 km s ^-1 . On the inner edge of the western part of the H α filament, there is a cooler filament emiting in the molecular hydrogen line ( λ=2.12 μ m) and in the dust continuum. In the northeastern part of the studied region, a new Herbig–Haro object is discovered and assigned the number HH 1232. The presence of several more new emission nebulae is suspected. The electron density in the studied regions of the H α filament, as well as HH 393 and HH 1232 N_e≲ 100 cm ^-3 . Arguments are presented in favor of the fact that the dusty disk wind has created a gas-and-dust nebula around ZZ Tau IRS, along the symmetry axis of which there is a jet moving in the direction of HH 393.

Источник BL Lac демонстрировал повышенную активность с августа 2020 до июля 2022 г. Максимум этой активности пришелся на период июль–август 2021 г. В этот период источник достиг исторических максимумов яркости в различных энергетических диапазонах. Наблюдения этого нестандартного максимума проводились сотрудниками ГАИШ Крымской станции ГАИШ и на телескопах Кавказской горной обсерватории (КГО ГАИШ МГУ). Были получены данные фотометрических наблюдений в оптическом диапазоне и ближней инфракрасной области, а также поляриметрические данные в оптике. При исследовании корреляции оптического и гамма-излучения, оптического и ИК-излучения подтвердилась ее высокая степень с почти нулевым запаздыванием – это значит, что области излучения пространственно совпадают либо расположены очень близко друг от друга. Степень линейной поляризации в исследуемый период менялась в широких пределах, достигая значения 20 \({\%}\) , причем наблюдалась антикорреляция степени линейной поляризации с яркостью объекта. Подобный характер изменения наблюдался и в предшествующие вспышки этого объекта. Изменения направления вектора напряженности электрического поля (EVPA) показывают зависимость от скорости изменения яркости объекта. Угол вектора напряженности электрического поля сильно менялся при вспышках, а в моменты медленного изменения интенсивности менялся медленно.

The source BL Lac exhibited an enhanced activity from August 2020 to July 2022. This activity peaked in July–August 2021. In this period the source reached historic brightness maxima in various energy ranges. Observations of this nonstandard maximum were carried out by the SAI staff of the Crimean Station of SAI and with the telescopes of the Caucasus Mountain Observatory (CMO SAI MSU). Optical and near-infrared photometric data as well as optical polarimetric data were obtained. When investigating the correlation between the optical and gamma-ray fluxes and between the optical and infrared fluxes, its high degree with an almost zero lag was confirmed—this means that the emission regions spatially coincide or are very close together. The degree of linear polarization in the investigated period changed in a wide range, reaching 20 % , with an anticorrelation between the degree of linear polarization and the brightness of the object having been observed. A similar pattern of variation was also observed in the preceding flares of this object. The changes in the electric vector position angle (EVPA) show a dependence on the rate of change of the object’s brightness. The EVPA changed greatly during flares and slowly at the times of a slow change in the intensity.

We present the results of the optical identification and spectroscopic redshift measurements of216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observationswere performed in 2020–2023 with the 6-m BTA telescope at the Special Astrophysical Observatory ofthe Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of theSternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope atthe Sayan Solar Observatory of the Institute of Solar–Terrestrial Physics of the Siberian Branch of theRussian Academy of Sciences, and the 1.5-m Russian–Turkish telescope (RTT-150) at the TU¨ BI˙ TAKObservatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements havebeen obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in theSRG/eROSITA survey and 22 galaxy clusters are at redshifts zspec 0.7, including three at zspec 1.Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters atzspec 0.7. For these observations we chose the most massive clusters and, therefore, most of the galaxyclusters presented here with the spectroscopic redshifts measured by us will most likely enter in future intothe cosmological samples of galaxy clusters from the SRG/eROSITA survey.

The four completed six-month sky surveys with the eROSITA telescope onboard the SRG orbital observatory allow tidal disruption events (TDEs) in galactic nuclei to be searched for by their X-ray variability. In this case, variable active galactic nuclei (AGNs) are detected in much larger quantities, for the elimination of which fairly stringent criteria have to be used. Some TDEs can be missed or misclassified as probable AGNs. Optical spectroscopy needs to be performed for the final identification of TDEs among the extragalactic SRG/eROSITA transients. We consider a set of criteria by which TDEs can be distinguished from AGNs based on optical spectra and photometric information. In particular, we propose to use the ratio of the X-ray luminosity to the [O III] 5007 Å luminosity. To test the proposed method, we consider a sample of 15 extragalactic SRG/eROSITA transients whose X-ray flux changed by more than a factor of 7 between two adjacent surveys. Spectra have been taken for all these objects with Russian optical telescopes: RTT-150, AZT-33IK, RC2500, and BTA. We have managed to reveal five new and one previously known TDEs and to classify seven sources as AGNs. The nature of two more transients remains in question. The proposed method will help to set priorities to obtain a maximally complete and reliable sample of TDEs in the SRG/eROSITA survey.