Massive clusters of galaxies are very rare in the observable Universe. Mergers of such clusters observed close to pericenter passage are even rarer. Here, we report on one such case: The massive (similar to 10(15) M-circle dot) and hot (kT similar to 10 keV) cluster CL0238.3+2005 at z=0.42. For this cluster, we combined X-ray data from SRG/eROSITA and Chandra, optical images from DESI, and spectroscopy from the BTA and RTT-150 telescopes. The X-ray and optical morphologies suggest an ongoing merger with a projected separation of the subhalos of similar to 200 kpc. The line-of-sight velocity of galaxies that are tentatively associated with the two merging halos differs by 2000-3000 km s(-1). We conclude that the merger axis is most likely neither close to the line of sight nor to the sky plane. We compare CL0238 with the two well-known clusters MACS0416 and the Bullet and conclude that CL0238 corresponds to an intermediate phase between the pre-merging MACS0416 cluster and the post-merger Bullet cluster. Namely, this cluster recently (only less than or similar to 0.1 Gyr ago) experienced an almost head-on merger. We argue that this "just after" system is a very rare case and an excellent target for lensing, the Sunyaev-Zeldovich effect, and X-ray studies that can constrain properties ranging from dynamics of mergers to self-interacting dark matter, and plasma effects in the intracluster medium that are associated with shock waves, for instance, electron-ion equilibration efficiency and relativistic particle acceleration.

The spectral and photometric studies of the cataclysmic variable Gaia 19cwm (or ZTF19aamkwxk) have been performed. Based on the analysis of long-term variability, it is concluded that the object belongs to WZ Sge type stars. The light curves show eclipses recurring with an orbital period of 86.32048± 0.00005 min, as well as an out-of-eclipse variability with a period of ≈6.45 min. The latter period is stable for ∼4 years and appears to correspond to the rotation of a magnetic white dwarf, i.e., Gaia 19cwm is an intermediate polar. The Gaia 19cwm spectra show photospheric lines of the white dwarf, and Doppler tomograms demonstrate the presence of an accretion disk and a hot spot. Analysis of the eclipse light curve gives an estimates of the white dwarf mass M_1=0.66± 0.06 M_⊙ , the donor mass M_2=0.073± 0.015 M_⊙ , and the orbital inclination i=83.8^∘± 1.1^∘ . Modeling of the spectral energy distribution gives the white dwarf temperature of T_eff≈ 13 000 K. The X-ray luminosity L_X=(1.6± 0.3)× 10^31 erg/s allows to assign Gaia 19cwm to a small group of low-luminosity intermediate polars.

We have carried out multiwavelength spectroscopic and photometric studies of the object SRGe J194401.8+284452 (2RXS J194401.4+284456, XMMSL2 J194402.0+284451, 2SXPS J194401.7+284450), the brightest X-ray source in the position uncertainty ellipse of the unidentified gamma-ray source 4FGL J1943.9+2841, with the goal of determining its nature and the possible association with the gamma-ray source. The object is shown to be a cataclysmic variable with an orbital period of about 1.5 h and clear evidence for the presence of an accretion disk around the white dwarf. It can be classified by its properties as an intermediate polar whose association with the gamma-ray source is unlikely. SRGe J194401.8+284452 exhibits abrupt transitions between its high and low luminosity states simultaneously in the optical and X-ray bands, which remain relatively stable on time scales of several months/years. This may be related to the change in the accretion rate by an order of magnitude. We have obtained constraints on the mass (0.3–0.9 M_⊙ ) and temperature ( 14 750± 1250 K) of the accreting white dwarf in the low state, the mass of the donor star ( ≤0.08± 0.01 M_⊙ ), and the orbital inclination of the binary system ( 40^∘-75^∘ ). In the low state we have detected an 8-min brightness variability in the optical band that is most likely related to the white-dwarf spin and not to the nonradial pulsations. In the high state we have revealed stochastic brightness variations on time scales of 1–15 min with amplitudes of 0.2– 0.6^m . SRGe J194401.8+284452 replenishes the small group of intermediate polars with the shortest orbital periods lying below the gap in the period distribution of these systems and exhibiting transitions between the states with high and low accretion rates. The brightness of the source at a level of 17-20^m in the 2000–8000 Å range and (5-50)× 10^-13 erg s ^-1 cm ^-2 in the range 0.3–10 keV makes it an interesting object for a detailed study of the physics of such systems.

We present the results of our optical study of the eclipsing polar Gaia23cer. We analyze the orbital brightness variability in high ( ⟨ r⟩≈ 16.5^m ) and low ( ⟨ r⟩≈ 19.2^m ) states. The system has an orbital period P_orb=102.0665± 0.0015 min and exhibits deep eclipses with a duration Δ t_ecl=401.30± 0.81 s. The spectra have a red cyclotron continuum with the Zeeman H α absorption triplet forming in a magnetic field with a strength B=15.2± 1.1 MG. The source of emission lines has a high radial velocity semi-amplitude ( K≈ 450 km s ^-1 ), and its eclipse lags behind the eclipse of the white dwarf. The mass M_1=0.79± 0.03 M_⊙ and temperature T=11 350± 650 K of the white dwarf have been estimated by modelling the spectral energy distribution. The eclipse duration corresponds to a donor mass M_2=0.10-0.13 M_⊙ and an orbital inclination i=84.3^∘-87.0^∘ . The donor temperature was estimated to be T≈ 2900 K by modelling the elliptical variability and eclipse depth.

Based on data from the ZTF photometric survey, we have revealed asynchrony of the polar SDSS J085414.02+390537.3. A beat period P_beat=24.6± 0.1 days, during which the system changes its brightness by ≈3^m , is distinguished in the light curves. Power peaks at the white-dwarf rotation period P_spin=113.197± 0.001 min and orbital period P_orb=113.560± 0.001 min are revealed in the periodograms, with the corresponding polar asynchrony being 1-P_orb/P_spin=0.3% . The photometric behavior of the polar points to a change of the main accreting pole during the beat period. Based on the Zeeman splitting of the H β line, we have estimated the mean magnetic field strength of the white dwarf to be B=28.5± 1.5 MG. The magnetic field strength near the magnetic pole has been found by modeling the cyclotron spectra to be B=34± 2 MG. The Doppler tomograms in the H β line exhibit a distribution of emission sources typical for polars in velocity space with evidence of the transition of the accretion stream from the ballistic trajectory to the magnetic one.

Massive clusters of galaxies are very rare in the observable Universe. Even rarer are mergers of such clusters observed close to pericenter passage. Here, we report on one such case: a massive ( 10^15 M_⊙) and hot (kT 10 keV) cluster CL0238.3+2005 at z≈ 0.42. For this cluster, we combine X-ray data from SRG/eROSITA and Chandra, optical images from DESI, and spectroscopy from BTA and RTT-150 telescopes. The X-ray and optical morphologies suggest an ongoing merger with the projected separation of subhalos of ∼ 200 kpc. The line-of-sight velocity of galaxies tentatively associated with the two merging halos differs by 2000-3000 km/s. We conclude that, most plausibly, the merger axis is neither close to the line of sight nor to the sky plane. We compare CL0238 with two well-known clusters MACS0416 and Bullet, and conclude that CL0238 corresponds to an intermediate phase between the pre-merging MACS0416 cluster and the post-merger Bullet cluster. Namely, this cluster has recently (only ≲ 0.1 Gyr ago) experienced an almost head-on merger. We argue that this "just after" system is a very rare case and an excellent target for lensing, Sunyaev-Zeldovich effect, and X-ray studies that can constrain properties ranging from dynamics of mergers to self-interacting dark matter, and plasma effects in intracluster medium that are associated with shock waves, e.g., electron-ion equilibration efficiency and relativistic particle acceleration.

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.

We have performed spectroscopic and photometric studies of the poorly explored polar BM CrB. Based on ZTF survey data, we have revealed three brightness states of the polar and evidence of the transition from one-pole to two-pole accretion as the average brightness of the system increases. We show that there are a change in the longitude of the main accretion spot (by $${\approx}17^{\circ}$$ ) and an increase in its extent (by $${\approx}10^{\circ}$$ ) when passing from the low state to the high one. Zeeman H $$\alpha$$ absorptions formed in a magnetic field of strength $$B=15.5\pm 1$$ MG are present in the spectra. The cool halo extending from the accretion spot to $${\approx}{1/4}$$ of the white dwarf radius can be the source of these absorptions. Our modeling of the behavior of the H $$\alpha$$ emission line shows that the main source of the emission is the segment of the accretion stream near the Lagrange point L $${}_{1}$$ that is periodically eclipsed by the donor star. The spectra exhibit a cyclotron component forming in the accretion spot. Their modeling by a simple accretion spot model gives constraints on the magnetic field strength, $$B=15$$ –40 MG, and the temperature, $$T_{e}\gtrsim 15$$ keV.

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.

Выполнены спектральное и фотометрическое исследования слабоизученного поляра BM CrB. На основе данных обзора ZTF выявлено три состояния блеска поляра и признаки перехода от однополюсного к двухполюсному режиму аккреции при увеличении среднего блеска системы. Показано, что при переходе от низкого состояния к высокому происходит изменение долготы главного аккреционного пятна (на \({\approx}17^{\circ}\) ) и увеличение его протяженности (на \({\approx}10^{\circ}\) ). В спектрах присутствуют зеемановские абсорбции линии H \(\alpha\) , которые формируются в магнитном поле напряженностью \(B=15.5\pm 1\) МГс. Источником этих абсорбций может быть холодное гало, простирающееся от аккреционного пятна на \({\approx}{1/4}\) радиуса белого карлика. Моделирование поведения эмиссионной линии Н \(\alpha\) показывает, что основным источником эмиссии является участок аккреционной струи вблизи точки Лагранжа L \({}_{1}\) , который периодически затмевается звездой-донором. В спектрах проявляется циклотронная компонента, формируемая в аккреционном пятне. Ее моделирование простой моделью аккреционного пятна дает ограничения на напряженность магнитного поля \(B=15\) –40 МГс и температуру \(T_{e}\gtrsim 15\) кэВ.

We present the results of our long-term photometric and spectroscopic monitoring with the1.5-m telescope RTT-150 for the optical counterpart of the high-mass X-ray binary IGR J21343+4738discovered in 2002 by the INTEGRAL space X-ray observatory. The X-ray source was also repeatedlydetected by the telescopes of the SRG observatory during the all-sky surveys in the period 2019–2021.We have investigated the spectroscopic and photometric variabilities of the optical counterpart, a Be star,caused by physical processes in the equatorial disk. We have analyzed the evolution of the equatorial diskparameters on a long time scale of 16 years.

We present the results of our long-term photometric and spectroscopic monitoring with the 1.5-m telescope RTT-150 for the optical counterpart of the high-mass X-ray binary IGR J21343+4738 discovered in 2002 by the INTEGRAL space X-ray observatory. The X-ray source was also repeatedly detected by the telescopes of the SRG observatory during the all-sky surveys in the period 2019–2021. We have investigated the spectroscopic and photometric variabilities of the optical counterpart, a Be star, caused by physical processes in the equatorial disk. The evolution of the equatorial disk parameters over a long time interval of 16 years has been analyzed.

In this work we performed a photometric and spectral study of the polar V379 Vir. We used the modeling of the IR light curves based on a simple model of cyclotrone radiation source, the method of synthetic photometry to fit the observed spectral distribution of the energy, as well as the modeling of the magnetic curve obtained from Zeeman splitting of the Hβ line to determine the parameters of the system. We managed to estimate the temperature of the white dwarf Teff = 11 450 K, the masses and radii of the primary and secondary components: M1 = 0.696 M , R1 = 0.011 R , M2 = 0.105 M , R2 = 0.14 R . The separation of the components was about 0.6 AU, and the inclination i lies in the range 47—60◦.