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We present the results of a UBVRcIc and JHKLM photometric monitoring campaign conducted in 2016–2020, low-resolution spectroscopy, and an analysis of the ASAS-3 and ASAS-SN photometric data for the semiregular variable and post-AGB candidate AU Vul (IRAS 20160+2734). The star is shown to experience quasi-periodic brightness oscillations with a variable amplitude and periods of 67–75 and 145–150 days in different time intervals. Its spectrum at maximum light is classified as that of an early G supergiant. In this spectrum H $$\alpha$$ , H $$\beta$$ , and higher Balmer lines are weakened, possibly, because of emission components, while the absorptions of s-process elements (Ba II, Sr II, and Y II) are slightly strengthened. An H $$\alpha$$ emission has been detected in the spectrum on the ascending branch of the light curve. TiO absorption bands are observed in the spectra at different brightness levels with maximum intensity at minimum light. The spectral energy distribution in the range 0.44 ( $$B$$ )–2.2 ( $$K$$ ) $$\mu$$ m corresponds to spectral types from G2I at maximum to G8I at minimum light. An emission excess has been detected in the $$L$$ and $$M$$ bands. The spectral energy distribution in the range 0.44–90 $$\mu$$ m constructed from our own observations and WISE, MSX, IRAS, and AKARI data is satisfactorily described by the sum of three components: a star and dust envelopes with $$T_{\textrm{hot}}=1000$$ K and $$T_{\textrm{cold}}=150$$ K. Using the distance based on the Gaia EDR3 parallax, $$D\approx 2300$$ pc, we have estimated the absolute magnitude, $$M_{V}\approx-4\overset{m}{.}45$$ , and the luminosity, $$L\approx 5450L_{\odot}$$ . A comparison with evolutionary models has shown that AU Vul may be at the very beginning of the post-AGB stage of evolution and have a mass $${\sim}0.55M_{\odot}$$ . We note that the star has a number of properties that distinguish it from typical post-AGB objects and make it akin to RV Tau stars.

We have performed a model analysis of the optical radiation from the young pre-cataclysmic variable Lan 30 and determined the set of its fundamental characteristics. Spectroscopic and multiband photometric observations have been carried out with the BTA and Zeiss-1000 telescopes at the Special Astrophysical Observatory of the Russian Academy of Sciences. The technique for modeling irradiated atmospheres of stars in close binary systems has been used for the computations of synthetic spectra and light curves. We have established the dominance of the radiation from an sdB subdwarf with atmospheric parameters $$T_{\textrm{eff}}=30\,500\pm 1100$$ K and $$\log g=5.60\pm 0.10$$ in the optical band under moderate influence of reflection effects on the light curves and HI line profiles. We have analyzed the measured sets of radial velocities of the primary component by taking into account the possible influence of reflection effects on them and refined their semi-amplitude $$K_{1}=56.3\pm 2.0$$ km s $${}^{-1}$$ . We have found that the nearly sinusoidal shape of the light curves of Lan 30 allows only an upper limit on the orbital inclination to be set. Based on agreement between the model and observed light curves, we have estimated the radii of the components at various inclinations. To determine their lower limit, we have used evolutionary estimates of the radii of low-mass main-sequence stars. The mass of the primary component has been taken to be equal to the normal mass of single sdB subdwarfs, $$M_{1}=0.47M_{\odot}$$ . As a result, we have determined the ranges of possible variations of the orbital inclination $$i=33^{\circ}{-}45^{\circ}$$ and semimajor axis, the radii of the components, and the mass $$M_{2}=0.127{-}0.175M_{\odot}$$ of the red dwarf.

The results of UBVRIJHK photometry for the type II-P supernova SN 2018aoq in NGC 4151 obtained from April 4, 2018, to January 14, 2019, with several telescopes, including the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, are presented. The main parameters of its light curves have been determined. The light and color curves of SN 2018aoq and several well-studied SNe II-P are compared. SN 2018aoq is shown to be intermediate in peak luminosity between SNe II-P with normal and reduced luminosities. Radiation-hydrodynamics simulations of the light curves and velocities of the expanding envelope of SN 2018aoq have been performed using the STELLA code. The best agreement with the observations has been achieved for SN progenitor models with an ejecta mass $$M=14{-}19M_{\odot}$$ , a mass of radioactive $${}^{56}$$ Ni $$M_{{}^{56}{\textrm{Ni}}}=0.01M_{\odot}$$ , and an explosion energy $$E=(0.4{-}0.6)\times 10^{51}$$ erg at metallicity $$Z=0.1Z_{\odot}$$ . The SN progenitor radius depends on the adopted distance $$D$$ to NGC 4151 and its values are $$R_{0}=500R_{\odot}$$ at $$D=20$$ Mpc and $$R_{0}=400R_{\odot}$$ at $$D=16$$ Mpc.

The plasma opacity in stars depends mainly on the local state of matter (the density, temperature, and chemical composition at the point of interest), but in supernova ejecta it also depends on the expansion velocity gradient, because the Doppler effect shifts the spectral lines differently in different ejecta layers. This effect is known in the literature as the expansion opacity. The existing approaches to the inclusion of this effect, in some cases, predict different results in identical conditions. In this paper we compare the approaches of Blinnikov (1996) and Friend and Castor (1983)–Eastman and Pinto (1993) to calculating the opacity in supernova ejecta and give examples of the influence of different approximations on the model light curves of supernovae.

We have carried out a systematic analysis of the light curves of gamma-ray bursts (GRBs) detected in the SPI-ACS experiment onboard the INTEGRAL observatory to search for extended emission. The emission occasionally recorded after the prompt active phase of a GRB in the form of an emission that is longer than the active phase and less intense is called the extended one. Out of the 739 brightest GRBs recorded from 2002 to 2017, extended emission has been detected in $${\sim}20\%$$ of the individual light curves; its maximum duration reaches 10 000 s. Two different types of extended emission have been revealed. One of them is an additional component of the light curve, which is described by a power law (PL) with an index of $$\alpha\sim{-}1$$ close to the PL index of the afterglow in the optical and X-ray bands. The second type can be described by a steeper PL decay of the light curve typical of the active burst phase. Extended emission has also been found in the combined light curve of long GRBs in the individual curves of which no extended emission has been detected. The PL index of the extended emission in the combined light curve is $$\alpha\sim{-}2.4$$ . It is most likely associated with the superposition of light curves at the active phase; its total duration is 800 s.

The emission from the Crab nebula exhibits a significant gamma-ray variability. In this paper we have analyzed this variability in terms of periodicity. Using the pulsar ephemeris constructed from radio data, we have obtained the light curve of the Crab nebula with the suppressed contribution of the pulsar in the energy range 100–300 MeV from August 2008 to March 2020. Using analysis of variance, we have found evidence for a periodicity of flares in the nebula with a period of 49 weeks. The statistical significance of the signal is 0.02 $$(2.1\sigma)$$ if only the highest peak in the periodogram is taken into account and 0.003 $$(2.7\sigma)$$ if two peaks, the highest one and the peak at twice the period (98 weeks) that can be caused by the same signal, are taken into account. If the periodicity found is real, then one should expect new periods of enhanced activity of the nebula in August 2021 and 2022.

Abstract We have studied the light curves of type Ia supernovae (SNe Ia) and the physical parameters inferred from them. We have constructed both analytical and numerical light curves of SNe Ia. Using an empirical relation between the SN luminosity and light-curve parameters, we have managed to impose constraints on the hydrodynamic solutions obtained by the STELLA code and to produce a sample of models that describe the observational properties of real SNe maximally accurately. With this sample we have established a relationship between the opacity in SN Ia ejecta and the parameters being determined directly from observations. The method has been tested on two classical SNe Ia as an example: 2011fe and 2012fr. The presented approach allows the opacity to be found without resorting to time-consuming computations.

Multicolor photometric observations in the optical band and a comprehensive study in the X-ray, gamma-ray, and radio bands are presented for GRB 181201A and its afterglow. The optical observations began $${\sim}0.5$$ day after the burst and lasted almost continuously for $${\sim}24$$ days. They were resumed after eight months, which allowed us to determine the contribution of the host galaxy to the measured fluxes and to estimate the related extinction. Such complete coverage of the light curve became possible owing to the coordinated work of a network of nine telescopes worldwide. Convincing evidence of an incipient supernova explosion at the location of the burst source was obtained at the end of the first series of observations. Thus, GRB 181201A became yet another event that confirmed the association of gamma-ray bursts with supernovae. Thirty such events based on photometric observations of burst afterglows were known before it. A comparison of the supernova-induced excess emission in the light curve of the afterglow from GRB 181201A with other events has allowed some of the supernova parameters to be determined.

In 2012–2014 we obtained 3296 CCD frames in the $$BVI_{c}$$ photometric system for the RR Lyrae star T Men with the 76-cm telescope of the South African Astronomical Observatory (SAAO, SAR) and the 1-m telescopes of the Las Cumbres Observatory Global Telescope (LCOGT). Our observations showed a slightly increased scatter of data points on the light curve, which allowed the Blazhko effect with a period $${\sim}18\overset{\textrm{d}}{.}49$$ to be detected. To study the variability of the pulsation period, we used all the available observations, including 1325 magnitude estimates from the digitized Harvard Astronomical Plate Collection (the DASCH project). This allowed us to construct an $$O-C$$ diagram spanning a time interval of 125 years and for the first time to detect at least three abrupt changes in the pulsation period.

We present the results of our analysis of the photospheric and chromospheric activity of the star DS Tuc (DS Tuc A, HD 222259A), which is a member of the Tucana–Horologium (Tuc–Hor) group with an age of 45 Myr. The star DS Tuc may be considered as the prototype of a young Sun, while its planetary system may be considered as the prototype of a young Solar system. Using data from the archive of TESS observations, we have analyzed the activity of this star. According to our estimate, the stellar rotation period is $$P=2.85\pm 0.18$$ days, which coincides with its known estimates. The light curve has been analyzed by the method of solving the inverse problem of reconstructing the maps of surface temperature inhomogeneities. We show that there are concentrations of spots at two longitudes on these maps. The spot positions undergo changes. The fraction $$S$$ of the spotted stellar surface is about 3.3 $$\%$$ . We have studied the object’s position on the $$S$$ –age, $$S$$ –rotation period, and $$S$$ –Rossby number diagrams and concluded that it corresponds to the general pattern of the dependences established previously for young stars with planetary systems and solar-type stars. Based on All Sky Automated Survey observations, we have estimated for the first time the possible activity cycle of DS Tuc to be 1610 days (4.4 years).

$$UBVJHKLM$$ photometry for the carbon Mira star V CrB are presented. The infrared observations were carried out in the time interval 1989–2018, while the $$U$$ , $$B$$ , and $$V$$ data were obtained in 2001–2014. The light and color curves are analyzed. The pulsation period of V CrB has been found to be $$355\overset{\textrm{d}}{.}2$$ in the infrared $$JHKLM$$ bands and $$352^{\textrm{d}}$$ for the optical $$BV$$ band. In the $$JHK$$ bands, apart from periodic pulsations, there are distinct sinusoidal variations in the average brightness level with a characteristic period of $${\sim}8300$$ days. Color–magnitude relationships have been revealed for the infrared and optical bands. The phase curves exhibit the wavelength dependence of the brightness variability amplitude. The light curves for various bands and colors are discussed. We have constructed the model of a spherically symmetric circumstellar dust envelope that allows the observed spectral energy distribution at both maximum and minimum light to be reproduced equally well (within the model assumptions) and is consistent with the observations of V CrB by differential speckle polarimetry. The model is characterized by the following parameters: the optical depth is $$\tau_{K}=0.33$$ , the inner and outer radii of the envelope are 8 and 40 000 AU, respectively. The envelope contains spherical carbon dust grains ( $$3/4$$ by mass) and silicon carbide dust grains. Dust grains with a radius of 0.5 $$\mu$$ m account for $$90\%$$ of the envelope mass. The remaining $$10\%$$ of the mass is accounted for by finer dust with a grain radius of 0.1 $$\mu$$ m. Based on the observational data, we have estimated the bolometric flux from V CrB: $$2.6\times 10^{-7}$$ and $$5.1\times 10^{-7}$$ erg cm $${}^{-2}$$ s $${}^{-1}$$ at minimum and maximum light, respectively. The effective temperature of the star is $$T_{\textrm{max}}=3000$$ K at maximum light and $$T_{\textrm{min}}=2400$$ K at minimum light.

An analysis of the spectra for the anomalous supernova ASASSN-15nx with a linear light curve has ruled out the emission mechanisms through radioactivity and shock interaction with circumstellar gas. An alternative mechanism for the emission of ASASSN-15nx based on the interaction of a rotating neutron star magnetosphere with gravitationally bound ejecta material is proposed. In the regime of stationary accretion the neutron star rotation frequency and the rotational energy losses decrease exponentially with time, which can explain the linearity of the light curve. Modeling of the light curve at the initial stage of the luminosity rise in combination with the expansion velocity suggest a low ejecta mass, ~1 M⨀. The shape of the [O I] 6300, 6364 A doublet profile points to asphericity of the oxygen distribution, which, in turn, provides evidence for a highly aspherical explosion.

Results of radar and photometric observations of the asteroid 2017 VR12 are presented. The observations were carried out during the asteroid’s close approach to the Earth in 2018 on March 5 with 32-m radio telescopes at the Zelenchukskaya and Badary Observatories and on March 6 with a 0.65-m optical telescope at the Ussuriysk Astrophysical Observatory. The asteroid’s radar observations were performed in the bistatic mode at a frequency of 8.56 GHz in combination with the 70-m Goldstone antenna. The echo power spectra for the asteroid were obtained as a result of the observations. The poleon silhouette breadth of this asteroid is 138 ± 14 m. We have estimated the radar albedos, 0.32 ± 0.04 and 0.31 ± 0.04, and the circular polarization ratios, 0.36 ± 0.02 and 0.34 ± 0.01, for the Zelenchukskaya and Badary Observatories, respectively. Based on our photometric observations, we have obtained the asteroid’s light curve, determined its rotation period, 1.378 ± 0.03 h, and constructed a two-dimensional surface contour.

The galaxy Mrk 501 has been monitored with the GT-48 Cherenkov telescope at the Crimean Astrophysical Observatory for 14 years (1997–2010). Very-high-energy (VHE) gamma-ray photons have been recorded from Mrk 501 with a total significance of 15σ. The arrival directions of VHE gamma-ray photons have been mapped. They have been identified with Mrk 501. The TeV fluxes from Mrk 501 are shown to be highly variable. The light curves from the data of Cherenkov telescopes and the telescopes that observed Mrk 501 at high energies (MeV and GeV) and in the X-ray and optical bands are presented. Based on the GT-48 observations, we have constructed a TeV spectrum and determined the differential spectral index. The fluxes and spectral characteristics of Mrk 501 inferred from the GT-48 observations are consistent with the data of the MAGIC and H.E.S.S. Collaborations, respectively.

We present the results of our photometric UBV JHKL observations for the symbiotic star V1413 Aql obtained in 2012–2018. An analysis of the data has shown that inMay 2017 the system passed to a quiescent state with B − V ≈ 0ṃ 6 for the first time since 1993. It lasted no more than five months. The J − K color at the primary minimum of 2012 reached 1ṃ 5, which, given the interstellar reddening, corresponds to spectral type M5-M6 III of the cool component. A secondary minimum has been detected at φ ≈ 0.5 on the JK phase light curves constructed for the dates of observations with B ≥ 13.

We present the results of multicolor (UBVJHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude ({\Delta}V, Comment: 30 pages, 12 figures, Accepted for publication in Astronomy Letters

Supernovae are such bright objects that they can be observed even at high redshifts. Some types of such events, for example, type Ia (thermonuclear), have peculiarities of the light curve, which allows them to be used for cosmological applications. The light curve is determined by the details of the explosion dynamics and nucleosynthesis: in particular, it depends on the amount of iron-peak elements produced during the explosion. We discuss the burning processes in such objects and the peculiarities of turbulence simulations in them, which is needed for a proper hydrodynamic description of the explosion process. A direct nucleosynthesis calculation is performed for the temperature and density profiles derived in the available 3D hydrodynamic explosion simulations. We show that in the supernova progenitor model considered the calculated abundances of elements from carbon to iron-peak elements are in good agreement both with the observations and with the calculations of other authors. At the same time, no r-elements are produced even at the maximum neutron excess for this model (Ye ~ 0.47) due to the slow evolution of the density and temperature.

We present the results of our UBVRI photometry for the type II-P supernova SN 2017eaw in NGC6946 obtained fromMay 14 to December 7, 2017, at several telescopes, including the 2.5-m telescope at the CaucasusHigh-Altitude Observatory of the SAIMSU. The dates andmagnitudes atmaximumlight and the light-curve parameters have been determined. The color evolution, extinction, and peak luminosity of SN 2017eaw are discussed. The results of our preliminary radiation–gasdynamic simulations of its light curves with the STELLA code describe satisfactorily the UBVRI observational data.

We present the results of our analysis of the JEM-X/INTEGRAL data obtained from January 2003 to January 2015 aimed at searching for type I X-ray bursts from known and new bursters. Such bursts are caused by thermonuclear explosions on the surface of a neutron star. We have searched for bursts in the records of the count rate of the JEM-X detectors in the 3–20 keV energy band. We have separately reconstructed and analyzed the light curves of 104 X-ray bursters known to date based on the JEM-X data. A similar search for bursts was previously carried out in the 15–25 keV data from the IBIS/ISGRI telescope onboard the INTEGRAL observatory obtained in 2003–2009. We have continued to analyze the data from this telescope up until the observations in January 2015. The joint catalog of bursts detected by the two instruments includes 2201 events; their basic parameters are given. The large size of the sample of bursts makes it one of the most representative of the existing one and allows various statistical studies of bursts to be performed. In particular, we have constructed the dependence of the mean rate of type I bursts from bursters on the luminosity (accretion rate), revealed an appreciable burst rate from sources with a near-Eddington luminosity, and investigated the population of multiple bursts with a recurrence time much shorter than the time it takes for a critical mass of matter required for the initiation of an explosion to be accumulated on the neutron star surface. Almost all of the detected bursts are associated with already known bursters,we have found only one previously unknown burster, IGRJ17380-3749, in the archival data, and one more known, but poorly studied source, AX J1754.2-2754, has been identified as a burster. Several similar sources have previously been identified as bursters directly during the INTEGRAL observations.

The U BV photometry and low-resolution spectroscopy for the semiregular variable AI CMi, a candidate for post-AGB objects, performed in 1996–2016 and 2000–2013, respectively, are presented. The star showed multiperiodic brightness variations with an amplitude up to $$1\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 5$$ in the V band, a significant (up to $$0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 4$$ ) bluing of the B − V and U − B colors as the star faded, and a change of its spectrum from G5 I to K3–5 I, depending on its brightness. A possible long-term fading of AI CMi below $$8\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 5$$ in the period from May 2013 to early 2015 is observed in the light curve. The colors in this episode did not change the pattern of their unusual behavior with brightness. The main feature of the spectrum for AI CMi is the appearance and strengthening of TiO absorption bands as its brightness declines, which are atypical in the spectra of ordinary G5–K3 supergiants. The bluing of the B − V and U − B colors is interpreted as the blanketing of stellar radiation predominantly in V (and to a lesser extent in B) by the TiO absorption bands whose intensity increases dramatically with decreasing brightness. Another cause of the bluing can be the scattering of stellar radiation by small dust particles in the gas–dust shell of AI CMi. The star’s continuum-normalized spectra over the period from 2000 to 2013 in the wavelength range 4200 to 7700 or 9200 A are presented. These were taken at different phases of the pulsation cycle and clearly demonstrate the behavior of the TiO absorption bands depending on the V magnitude and B − V color. The equivalent widths of individual TiO bands weremeasured, and their correlation with the photometric parameters of the star is shown. AI CMi belongs to the O-rich branch of AGB/post-AGB supergiants and has a luminosity of ~4000 L ⊙ at a distance of 1500 ± 700 pc. The mass of AI CMi is most likely small and close to the lower mass limit for post-AGB stars. The connection of the star’s pulsational activity and nonstationary wind with the formation of its molecular and dust shells is discussed briefly.

During the scanning observations of the Galactic Center region in August - September 2016 we detected the new outburst of the historical X-ray nova GRS 1739-278, the black hole candidate LMXB system. In this letter we present results of INTEGRAL and Swift-XRT observations taken during the outburst. In hard X-ray band (20-60 keV) the flux from the source raised from $\sim$11 to $\sim$30 mCrab between 3 and 14 of September. For nearly 8 days the source has been observed at this flux level and then faded to $\sim$15 mCrab. The broadband quasi-simultaneous spectrum obtained during the outburst is well described by the absorbed powerlaw with the photon index $\Gamma=1.86\pm0.07$ in broad energy range 0.5-150 keV, with absorption corresponding to ${N_{H}}=2.3\times10^{22}$ cm$^{-2}$ assuming solar abundance. Based on this we can conclude that the source was in the low/hard state. From the lightcurve and spectra we propose that this outburst was `failed', i.e. amount of accreted matter was not sufficient to achieve the high/soft spectral state with dominant soft blackbody component as seen in normal outbursts of black hole candidates., Comment: Accepted for publication in Astronomy Letters, 2017, 43, 3

We have analyzed the light curves of 519 short gamma-ray bursts (GRBs) detected in the SPI-ACS/INTEGRAL experiment from December 2002 to May 2014 to search for precursors. Both the light curves of 519 individual events and the averaged light curve of 372 brightest bursts have been analyzed. In a few cases, we have found and thoroughly studied precursor candidates based on SPI-ACS/INTEGRAL, GBM/Fermi, and LAT/Fermi data. A statistical analysis of the averaged light curve for the entire sample of short bursts has revealed no regular precursor. Upper limits for the relative intensity of precursors have been estimated. No convincing evidence for the existence of precursors of short GRBs has been found. We show that the fraction of short GRBs with precursors is less than 0.4% of all short bursts., Comment: 33 pages, 11 figures

Based on five high-resolution spectra in the range 5625–7525 ˚A taken in 1995 and covering the ascending branch of the light curve from minimum to maximum, we have performed spectroscopic studies of the classical Cepheid ζ Gem. The atmospheric parameters and chemical composition of the Cepheid have been refined. The abundances of the key elements of the evolution of yellow supergiants are typical for an object that has passed the first dredge-up: a C underabundance, N, Na, and Al overabundances, and nearly solar O and Mg abundances. We have estimated [Fe/H] = +0.01 dex; the abundances of the remaining elements are also nearly solar. The metal absorption lines in all spectra show a clear asymmetry and the formation of secondary blue (B1 and B2) and red (R1 and R2) components, just as for the Cepheid X Sgr. The Hα absorption line is also split into blue (B) and red (R) components with different depths changing with pulsation phase. To analyze the velocity field in the atmosphere of ζ Gem, we have estimated the radial velocities from specially selected (with clear signatures of the B1, B2, R1, and R2 components) absorption lines (neutral atoms and ions) of metals (38 lines) and the B and R components of the Hα line. Analysis of these estimates has shown that their scatter is from −22 to 36 km s−1 for all pulsation phases but does not exceed 35–40 km s−1 for each individual phase, while it does not exceed 22 km s−1 for the Hα line components. The radial velocity estimates for the metal lines and their B1 and B2 components have been found to depend on the depths, suggesting the presence of a velocity gradient in the atmosphere. No significant difference in velocities between the atoms and ions of the metal lines is observed, i.e., there is no significant inhomogeneity in the upper atmospheric layers of the Cepheid. Since the averaged radial velocity estimates for the cores of the metal lines and their B1 and B2 components change with pulsation phase and coincide with those for the B component of the Hα line, they are all formed in the Cepheid’s atmosphere. The formation and passage of a shock wave due to the κ-mechanism at work can be responsible for the stronger scatter of the B1 and B2 components in their velocities at phases after the Cepheid’s minimum radius. The averaged velocities of the R1 components also change with pulsation phase and differ only slightly from the remaining ones. On the other hand, the mean velocity estimate for the R component of the Hα line at all phases is +32.72 ± 2.50 km s−1 and differs significantly from the bulk of the velocities, suggesting the formation of this component in the envelope around the Cepheid. The unusual behavior of the mean velocities for the R2 components of the metal absorption lines can also point to their formation in the envelope and can be yet another indicator of its presence around ζ Gem.

In this work the light curves (LCs) solutions along with the radial velocity curve of the semidetached binary systemXZ And are presented using the PHOEBE program(ver 0.31a). Absolute parameters of the stellar components were then determined, enabling us to discuss structure and evolutionary status of the system. The analysis indicates that the primary is a non-synchronous (i.e., F1 = 3.50 ± 0.01) Main Sequence (MS) star and the secondary is a bit more evolved, and fills its Roche critical surface. In addition, times of minima data (“O − C curve”) were analyzed. Apart from an almost parabolic variation in the general trend of O − C data, which was attributed to a mass transfer from the secondary with the rate 2 = (9.52 ± 0.41) × 10−10 M ⊙ yr−1; two cyclic variations with mean periods of 34.8 ± 2.4 and 23.3 ± 3.0 yr, modulating the orbital period, were found, which were attributed to a third body orbiting around the system, and magnetic activity cycle effect, respectively.

The model of a nonstationary α-disk with allowance made for the irradiation and vertical convection in the outer accretion disk, where hydrogen is partially ionized, is considered. Allowance for the energy release through turbulent viscosity is shown to be important in the mixing-length convection theory in accretion disks. This model has been used to explain the optical and X-ray light curves of the X-ray nova A0620-00. The turbulent viscosity parameter of the accretion disk has been estimated, α = 0.5−0.6, to explain the luminosity decay rate on the descending branch of the X-ray light curve for the A0620-00 1975 outburst. The secondary luminosity maximum on the light curves is explained by assuming an additional injection of matter into the accretion disk from the optical companion.

Results of the study of the X-ray nova SWIFT J174510.8-262411 by the INTEGRAL, SWIFT, and VLA observatories in September–October 2012 at the early outburst phase are presented. Attention is focused on analyzing the power spectrum of X-ray flux fluctuations of the source in which a powerful quasi-periodic oscillation (QPO) peak has been detected. The evolution of the QPO peak parameters with time has been traced; a correlation between the QPO frequency, low-frequency noise (LFN) parameters, X-ray and radio fluxes, and fractional polarization of radio emission has been revealed. The fractal properties of the source’s light curves in the standard X-ray band (

The photoelectric UBV observations of the peculiar symbiotic star V1329 Cyg performed at the Crimean Station of the SAI-MSU during 245 nights over the period 2003–2014 are presented. The star’s light curves since 1973 from the Crimean observations are shown. The brightness decline after its outburst over the last 40 years was $$0_ \cdot ^m 8 - 1_ \cdot ^m 0$$ . The phase color curves at phases 0.2 and 0.8 have maxima. Their qualitative interpretation in terms of the model of interacting winds in symbiotic binary star systems is proposed. The orbital period of the binary system has been redetermined. The spectroscopic observations at the 125-cm telescope of the Crimean Station from 1994 to 2014 have confirmed the change in the system’s emission spectrum with orbital phase. The HI, He I, and Fe II line fluxes clearly trace the orbital motion. The Balmer hydrogen lines as well as the continuum at λ6000 and the V-band flux change by a factor of ∼3.5 from minimum to maximum light. The neutral helium lines change by a factor of 5. The high-excitation He II, [FeVII], [Ca VII] lines and the Raman O VI λ6825 line have shown changes in the fluxes by a factor of ∼2–3 weakly correlating with the orbital phase. The equivalent widths of the HI and He I lines are maximal at the star’s maximum light and have distinct minima at phases 0.2 and 0.8, while the equivalent widths of the He II, [FeVII], and [CaVII] lines are minimal in the range of phases 0.2–0.8. The question about the location of the permitted and forbidden line emission zones in the binary system V1329 Cyg is discussed. The evolution of the emission spectrum for V1329 Cyg from 1980 to 2014 has been studied on the basis of new and archival data. A gradual decrease in the absolute fluxes of the nebular emission lines has been detected. The [O III] and [Fe VII] lines have weakened significantly. However, the [Fe X] λ6375 A line has appeared and gradually strengthened, suggesting an increase in the degree of gas ionization in the line formation zone.

In 2013–2014, we obtained 14959 CCD frames in the BV Ic photometric system for 170 classical Cepheids from the General Catalogue of Variable Stars. We performed our observations with the 76-cm telescope of the South African Astronomical Observatory (SAAO, South Africa) using the SBIG ST-10XME CCD camera. The tables of observations, the plots of light curves, and the current light elements are presented.

Results of a fractal analysis of the X-ray light curves from accreting black holes in low-mass binary systems are presented for the object GX 339-4 as an example. The fractal dimension of the light curves is shown to be strongly dependent on the presence of quasi-periodic oscillations (QPOs) in the observations. A correlation between the fractal dimension of the light curves and the frequency of the QPO peak has been revealed. A method supplementary to a Fourier analysis that allows the pattern of accretion disk emission as a function of time scales to be investigated is proposed. The results of this analysis can be explained if the accretion disk is separated by the QPO region into two zones with different emission parameters.

Observations of the spotted rotating G8 V star HII 296, a member of the Pleiades cluster, are presented. The star has exhibited stable periodic brightness variations for several years. The apparent rotation period of the star in our observations was 2.53 and 2.58 days. The shape and amplitude of its light curve changed from year to year. During our observational monitoring from September to November 2012, we recorded an almost complete absence of brightness variations in September. A rapid (within a few weeks) and manyfold increase in the brightness variability amplitude was observed in October and November 2012. The changes in the rotation period, amplitude, and shape of the light curve suggest an intense relative motion of spots in latitude and longitude.

In 2008–2013, we obtained 11333 CCD BVIc frames for 57 Cepheids from the General Catalogue of Variable Stars. We performed our observations with the 76-cm telescope of the South African Astronomical Observatory (SAAO, South Africa) and the 40-cm telescope of the Cerro Armazones Astronomical Observatory of the Universidad Catolica del Norte (OCA, Chile) using the SBIG ST-10XME CCD camera. The tables of observations, the plots of light curves, and the current light elements are presented. Comparison of our light curves with those constructed from photoelectric observations shows that the differences between their mean magnitudes exceed 0ṃ05 in 20% of the cases. This suggests the necessity of performing CCD observations for all Cepheids.

The properties of the hot zone in the accretion flow near the surface of a magnetized white dwarf have been studied. For this purpose, the aperiodic optical variability of LS Peg, one of the brightest intermediate polars in the northern sky, has been investigated. The main radiation of the hot zone, which is then reemitted in the optical band, results from the radiation of an optically thin plasma heated during the passage of the accretion flow of a standing shock. Recently, Semena and Revnivtsev (2012) have shown that the aperiodic variability (flickering) of accreting magnetized white dwarfs should have a characteristic feature in the range of Fourier frequencies corresponding to the plasma cooling time in this hot region. The photometric brightness measurements for LS Peg made with the RTT-150 telescope using a high-speed ANDOR iXon CCD array have allowed the break frequency in the power spectrum of the source’s variability to be constrained. Constraints on the geometry of the accretion column for the white dwarf in LS Peg and on the plasma parameters in it have been obtained.

Exoplanet observations have been performed on the automated Pulkovo Observatory telescopes. We have obtained 33 transit light curves for 16 known exoplanets and six transit observations for three exoplanet candidates discovered by the Kepler telescope. Based on our observations, we have reliably confirmed the existence of an exoplanet with an extremely large radius, Rpl = 1.83 ± 0.16RJup, in the system KOI 256 and detected a strong deviation of its orbital revolution from the theoretically predicted one. During the transit of the exoplanet WASP-12b across the stellar disk, we detected bursts that could be caused by the planet transit across spots on the star or by the presence of a satellite around this exoplanet. We detected possible periodic variations in the duration of the exoplanet transit across the stellar disk with time for HAT-P-12b that could be caused by variations in orbital inclination. The transit duration and depth, the central transit time, and the radius and orbital inclination of the planet have been estimated. The equilibrium temperature and albedo have been estimated for several exoplanets.

In course of the search for the optical identifications associated with ROSAT X-ray sources we have found a highly variable object with the very unusual long-term behavior, color indices and high X-ray-to-optical flux ratio. We report the archival photometric light curve from the Catalina Sky Survey, optical spectroscopy from RTT150 and time-resolved photometry from Astrotel-Caucasus telescope. The object appears to be the magnetic cataclysmic variable (polar) with orbital period of P=3.20 hr., 8 pages, 5 figures. Submitted to Astronomy Letters

We show that the set of observational characteristics for low-mass X-ray binaries in the optical and X-ray bands can be explained in terms of the model of an optically thick accretion disk with an atmosphere irradiated by a central X-ray source. We show that this set of observational data can be successfully used to measure the orbital inclination of a binary, the geometric parameters of its accretion disk, and the reprocessing time of X-emission to optical one. For the burster GS 1826-238, a low-mass X-ray binary with a neutron star, we have estimated the binary inclination and the thickness of the disk atmosphere at the outer edge from the mean optical flux and the amplitude of periodic modulations in the optical light curve: i = 62.5° ± 5.5° and H d/R d = 0.145 ± 0.009. The optical response time of the binary to an X-ray burst disagrees with the geometric delay in the propagation of X-ray photons in the binary. We believe that this points to a finite X-ray reprocessing/reradiation time, 1.0 s ≲ τ repr ≲ 2.2 s, in the hot atmosphere above the accretion disk.

Based on two high-dispersion spectra of the close binary BW Boo, we have detected lines of the secondary component whose contribution to the combined spectrum does not exceed 2%. We have determined the rotation velocities of the components and spectroscopic orbital elements. Numerous lines of neutral and ionized iron have been used to determine the effective temperature and surface gravity for the primary component. The photometric light curves for this binary have been solved for the first time. Its primary component is an A2Vm star with a mass of 2 ± 0.1M ⊙ and a radius of 1.9 ± 0.4R ⊙. Its rotation velocity is 2 km s−1, which is a factor of 18 lower than the pseudo-synchronous velocity for this component. The G6 secondary component, a T Tau star, has a rotation velocity of 17 km s−1, amass of 1.1M ⊙, and a radius of 1 R ⊙. The age of the binary has been estimated to be 107 yr.

Long-term photometric and spectroscopic observations of the yellow symbiotic star LT Del are analyzed. UBV light curves are presented. Based on the observations of 20 cycles, we have refined the orbital period of the star, P = 476 · d 0 ± 1 · d 0. The brightness has been found to be unstable at some orbital phases with an amplitude up to 0 · 3. We have measured the fluxes in hydrogen and helium emission lines and in continuum and investigated their relationship to the orbital period. The fluxes in hydrogen and HeI lines follow the UBV light curves in phase; the He II 4686 A flux does not depend on the phase and is constant within the accuracy of our measurements. The intensity ratio of the 4686 A andHβ lines changes from 0.2 to 0.9 over the period. We interpret the spectroscopic observations based on the hypothesis of heating and ionization of the stellar wind from a cool component by high-frequency radiation from a hot star with a temperature of 105 K. We have estimated the spectral type of the cool star from our photometry and its continuum energy distribution as a bright K2–4 red giant branch halo star. The bolometric luminosity and mass loss rate have been estimated for the K component to be L bol ∼ 700L ⊙ and $\dot{M}$ ∼ 10−8 M ⊙ yr−1, respectively.

We study the optical variability of the peculiar Galactic source SS 433 using the observations made with the Russian Turkish 1.5-m telescope (RTT150). A simple technique which allows to obtain high-quality photometric measurements with 0.3-1 s time resolution using ordinary CCD is described in detail. Using the test observations of nonvariable stars, we show that the atmospheric turbulence introduces no significant distortions into the measured light curves. Therefore, the data obtained in this way are well suited for studying the aperiodic variability of various objects. The large amount of SS 433 optical light curve measurements obtained in this way allowed us to obtain the power spectra of its flux variability with a record sensitivity up to frequencies of ~0.5 Hz and to detect its break at frequency =~2.4e-3 Hz. We suggest that this break in the power spectrum results from the smoothing of the optical flux variability due to a finite size of the emitting region. Based on our measurement of the break frequency in the power spectrum, we estimated the size of the accretion-disk photosphere as 2e12 cm. We show that the amplitude of the variability in SS 433 decreases sharply during accretion-disk eclipses, but it does not disappear completely. This suggests that the size of the variable optical emission source is comparable to that of the normal star whose size is therefore R_O \approx 2e12 cm \approx 30 R_sun. The decrease in flux variability amplitude during eclipses suggests the presence of a nonvariable optical emission component with a magnitude m_R=~13.2., 12 pages, 11 figures. Accepted for publication in Astronomy Letters. The original version in Russian is available at http://hea.iki.rssi.ru/rtt150/ru/ss433_pazh10/pss433_fast.pdf

Based on our UBV RI observations and X-ray data from the RXTE satellite, we have investigated the variability of the galaxy NGC 7469 over the period 1995–2009. In 1995–2000, the optical brightness of the galactic nucleus changed almost by 1 m in the U band. In 2000–2009, the amplitude of the optical variations was considerably lower. Regular X-ray observations began only in 2003. The X-ray fractional variability amplitude is higher than the optical one. The optical variability amplitude decreases with increasing wavelength. The full width at half maximum of the X-ray and B-band autocorrelation functions is about 8 and 62 days, respectively. The structure functions (SF) in the X-ray range on time scales up to 7 days and in the optical range on time scales up to 100 days have the form of a power law SF(τ) ∼ τ b , where τ is the time shift. On time scales of more than a day, where both structure functions have been determined rather reliably, their slopes differ markedly: b = 1.34 ± 0.06 and b = 0.25 ± 0.05 for the optical and X-ray ranges, respectively. The X-ray and B-band structure functions begin to flatten, respectively, near 6–8 days and on time scales of about 90 days. The observed structure functions can be described by the model of a superposition of independent Gaussian flares whose number changes with duration ω as n(ω) ∼ ω α and whose amplitudes depend on duration as A(ω) ∼ ω β. The flux distribution and the flux-amplitude relation are consistent with the model of a light curve in the form of a superposition of random flares. Once the fast intensity variations have been filtered out on long time scales, the X-ray light curve correlates well with the optical one. No lag of the X-ray variations relative to those in the B band is detected. The light variations in the R and I bands lag behind those in the B band calculated from the centroid of the cross-correlation function by 2.6 and 3.5 days, respectively, at a 3σ confidence level.

Using visual, photographic, and photoelectric measurements, we have constructed a historical light curve for the young binary system UY Aur on an interval longer than 100 yr. About a quarter of all magnitude estimates have been obtained for the first time from photographic plates of the Sternberg Astronomical Institute and Harvard College Observatory Astronomical Plate Stacks. Analysis of the light curve and the magnitude dependences of the polarization and color has led us to the following conclusions. Cyclic variations in the seasonally mean brightness of the binary’s primary component UY Aur A with a period of ≃16.3 yr occurred from the mid-1920s to the mid-1940s and after 1986. The variations are caused by the change in the rate of disk accretion onto the star attributable to the motion of the hypothetical companion UY Aur C around the primary star in an orbit with a semimajor axis of ≃ 6 AU. From the early 1950s to the mid-1980s, the periodicity of the seasonally mean variations was not noticeable due to nonperiodic eclipses of UY Aur A by gas-dust clouds. Between 1945 and 1974, another gas-dust cloud obscured and still obscures the component UY Aur B, causing its mean optical brightness to drop by several magnitudes. The role of the clouds that caused an almost simultaneous eclipse of the stars, whose separation in projection onto the celestial sphere exceeds 100 AU, is played by the denser and puffed-up regions of their accretion disks. These regions are the result of a dynamical interaction between the binary’s stars and the outer circumbinary accretion disk. The extinction variations with time are attributable to orbital motion of the binary’s stars and azimuthal inhomogeneity of the clump regions in the disks. A number of observational tests are suggested to verify our conclusions.

Recently, it has been shown that the cataclysmic variable EI UMa should be attributed to intermediate polars on the basis of its X-ray spectrum. However, no short-period oscillations with a high degree of coherence typical of intermediate polars could be detected in this star. Therefore, it was suggested to call it a “hidden” intermediate polar. Using a multichannel photometer and a 70-cm telescope, we performed observations of EI UMa for 60 h. Analysis of the observations has clearly revealed a brightness oscillation with a period of 769.63±0.10 s and a mean semiamplitude of 0·m008 completely coherent during one month of our observations that was detectable during each of the 13 observing nights. On the basis of this detection, EI UMa should be considered a classical intermediate polar rather than a hidden one. In addition, we have managed to detect the orbital variability of EI UMa for the first time. The orbital light curve is extremely interesting, showing evidence of partial eclipses of the accretion flow.

We present new photometric data and analysis of the long-duration UBV photoelectric observations for three candidates to protoplanetary objects - F-supergiants with IR-excesses located at large galactic latitudes, IRAS 18095+2704, IRAS 19386+0155, and IRAS 19500-1709. All three stars have revealed quasiperiodic low-amplitude variabilities caused by pulsations observed against the long-term trends of brightnesses. For IRAS 18095+2704=V887 Her we have found a pulsation period of 109 days and a linear trend of brightness under the constant colours if being averaged over the year timescale. The light curve of IRAS 19386+0155=V1648 Aql over 2000-2008 can be approximated by a wave with a main period of 102 days which is modulated by close frequency, with a period of 98 days, that results in brightness oscillations with a variable amplitude. V1648 Aql has also shown synchronous reddening together with a persistent rise of brightness in the V-band. IRAS 19500-1709=V5112 Sgr experiences irregular pulsations with the periods of 39 and 47 days. The long-term component of the variability of V5112 Sgr may be related to the binary character of this star.

The results of numerical simulations of light curves and spectra and the influence of special relativity effects at the epoch of supernova shock breakout are considered. The algorithm of the RADA code used in the simulations is described. Prospects for using the numerical simulation results to analyze and interpret available and future data from the SWIFT and LOBSTER space observatories are discussed.

Most of the extragalactic sources from which very-high-energy (VHE, E > 1011 eV) gamma-ray fluxes have been detected belong to the category of high-energy peaked BL Lacertae objects (HBLs)—the sources in which the synchrotron radiation peaks in the UV or X-ray band. They often have higher X-ray luminosities than the VHE gamma-ray energy output, which makes them the most valuable objects for studying the characteristic spectral and temporal variations in the region of the synchrotron peak of the spectral energy distribution. The blazar 1ES 1426+428 belonging to this category is a target of many multiwavelength studies, both orbital and ground-based ones. The properties of its X-ray emission have also been investigated using RXTE/PCA, XMM-Newton, and SWIFT observations. Archival PCA/RXTE data with a total exposure time in 2002 and 2004 of ≈120h and the most recent available background and calibration files have been used. The extracted light curves of 1ES 1426+428 in the 2.9–24 keV energy band have shown an intense flaring activity on various time scales. Analysis of the observational data has also confirmed the spectral hardening with increasing X-ray intensity typical of blazars. The flaring state of the object is also characterized by a flat spectrum, which steepens with decreasing flux. The previously detected evidence of a spectral hysteresis in a separate flare has also been confirmed. Observations of 1ES 1426+428 with the SWIFT/XRT telescope and the EPIC instrument onboard XMM-Newton have revealed several intermediate-intensity flares in the 1.5–12 keV energy band with flux variations reaching a factor of 2, while analysis of the light curves has revealed a correlation between two components of the X-ray emission from the object.

magnitude estimates. The maximum amplitude of the V -band light variations was >8 m . Six deep minima and four high maxima were observed. Analysis of the light curve has shown that the pulsation period of the star remained constant since 1991 and was P = 115 days. We have studied the wavelength dependence of the extinction at various phases of the light curve. The blueing of the B–V color at deep minima is interpreted as the result of light scattering in the circumstellar dust shell of the star formed by preceding dust ejections since 1992. Our spectroscopic observations performed on nine nights in 2003–2007 with the 125-cm telescope at the Crimean Station of the Sternberg Institute have confirmed the previously detected intensity variations of the Swan bands and the sodium doublet with brightness. It is noted that the Swan bands originate in the upper atmosphere, the star’s extended envelope, while the sodium doublet originates mainly in the circumstellar shell of FG Sge. We suggest that the star is currently located in the temperature–luminosity diagram at the turning point of the horizontal track of cooling in the direction of hot stars—evolution caused by the last helium shell flash at the planetary nebula stage.

In 2003–2008, highly accurate photoelectric and CCD observations of the close binary system DI Her were performed in the V band. The light curves of three primary and three secondary eclipses were constructed. These observations, along with the highly accurate photoelectric observations of other authors obtained in different years from 1963 to 1986, have confirmed the difference between the observed (1.o3 ± 0.o1/100 yr) and theoretical (4.o3/100 yr) rates of apsidal motion. Our photometric data are indicative of a possible variability in the system with period P′ = 1.175 days and amplitude A′ = 0.m011, which is probably related to the pulsations of one of the components. There may be a third body in the system that produces in-phase variations in the times of primary and secondary minima with a period of 10.5 yr and an amplitude of 1.m5.

Cepheids, the principal distance indicators, point to the existence of two inner Galactic spiral arms in the immediate solar neighborhood (within 5 kpc). However, the available Cepheids are clearly insufficient for a detailed study of the structure of these arms. Fortunately, southern-sky monitoring (the ASAS project) has led to the discovery of many new variable stars, more than one thousand of which have been identified as possible Cepheids. To improve the classification and to construct reliable light curves for new Cepheids, we have begun their regular photometric observations. Here, we present the first results: 1192 photoelectricBV Ic observations were performed for 33 classical Cepheidswith the 76-cm telescope of the South-African Astronomical Observatory (SAAO, the South-African Republic) from December 2006 to January 2008. We provide tables of observations and V light and B-V and V-Ic color curves. The new observations together with ASAS-3 data have been used to improve the elements of the light variations.

Observational parameters of the optical and gamma-ray emissions from 58 gamma-ray bursts (GRBs) with discovered afterglows and known redshifts are analyzed. The distributions of these parameters and pair correlations between them are studied. Approximately half of the objects exhibit a relatively slow decrease in the optical flux at initial afterglow phases (with a power-law index in the decay law α < 1). Correlations have been found between the luminosities, energies, and durations of the optical and gamma-ray emissions, which can be explained by the presence of universal features in the light curves. A correlation of the peak luminosity for afterglows with the redshift and an anticorrelation of their durations with the redshift have been found for the first time. Against the background of a weak z dependence of the total afterglow energy, this effect can be explained by cosmological evolution of the GRB environment, which determines the rate of optical energy release.

Results of the optical identification of the hard X-ray source IGR J08390-4833 recently discovered in the INTEGRAL all-sky survey are presented. We show that the source is most likely a cataclysmic variable, i.e., an accreting white dwarf in a binary. Analysis of its optical light curve clearly reveals intrinsic variability on timescales of the order of an hour or longer. However, the short time of the source’s optical observations does not allow a definitive conclusion about the periodicity of the detected variability to be reached. Further optical and X-ray observations are required for a more accurate classification of the source.