Preface

One of the most enigmatic groups of stars is the classical Be stars of the main sequence (MS), which include rapidly rotating stars of spectral class B with emission lines in the spectrum. Despite the more than 150-year history of studying these objects, their nature remains mysterious. The appearance of emission lines in the spectra of Be stars is associated with the formation of a circumstellar disk, the formation mechanisms of which are debated. Many authors consider the important role of the magnetic field in the formation of the disk, although up to date no magnetic field has been detected in any Be star. Among Be stars, a small (about 1%) group of gamma Cassiopeia type stars stands out. Specific features of these stars include high X-ray luminosity, intermediate between the luminosity of OB stars and that of X-ray binary stars with Be components, a hard spectrum with a flux ratio in the hard (2-10 keV) and soft (0.2-2 keV) energy bands greater than 1.6, and variability X-ray radiation, including on minute scales. Under the assumption that the X-ray spectrum of gamma Cas stars is completely thermal, the temperature of the plasma emitting in X-rays can reach 20-30 keV or more. There are several alternative models for the formation of the X-ray spectrum of gamma Cas stars, which indicates that the nature of the X-rays from this group of stars is hardly clear.

Now 25 gamma Cas stars and 2 candidates are known. Fifteen of the total 27 objects are available for observation in Russia. The present Project involves carrying out optical observations of gamma Cas type stars with high time resolution (less than a minute) to study the superfast variability of the line profiles in their visible spectra. It is planned to carry out such observations at the BTA, at the 2.5-m telescope of the KGO, and at the 1.25-m telescope of the Crimean station of the SAI. Studies of optical variability will help to understand how different and if the nature of the variability of gamma type Cas stars differs from the variability of other B and Be stars. To analyze the rapid photometric variability of gamma Cas stars, observational data from the TESS satellite will be used. Together with the analysis of optical variability, studies of the variability of X-ray light curves from the gamma Cas type stars will be carried out from an analysis of archival observations on the XMM-Newton and Chandra satellites. To study the relationship between X-ray and optical variability, we will also use the optical spectra of gamma Cas stars from the BESS database. Studying the correlation between X-ray and optical variability will allow us to investigate how differ the parameters of the regions where optical and X-ray radiation are formed. An important part of the proposed project will be the polarization observations of the bright gamma Cas type stars at the MSP spectrograph of the 6-m russian telescope BTA to determine the upper limit of the global magnetic field of these stars and looking for the local magnetic fields.

A new part of the Project will also be the expansion of the number of the known gamma Cas type stars. The archival observations on the XMM-Newton satellite with a low-resolution EPIC camera will be used for that.

The realization of the Project helps us to understand both the nature of mysterious gamma Cas type stars and the mechanisms of how their circumstellar disks are formed. This project will shed light on understanding similar mechanisms for other Be stars.

Results of work in the first year (2023)

Project's observations were made on January 12, 2023 under the on the 6-meter russian BTA telescope with the MSS spectrograph with a circular polarization analyzer. Spectra of 5 gamma type Cas stars were obtained: zet Tau, HD 45995, V558 Lyr, V2156 Cyg and V810 Cas (total of 112 spectra with exposures from 15 to 600 seconds.). Reducing of all obtained spectra was performed.
On nights of June 22 and 23, 2023, observations of the gamma Cas type star Cas V558 Lyr (HD183362) were made using the 1.25-m telescope of the Crimean station of the State Astronomical Institute. 650 low-resolution spectra of this star were obtained in the wavelength range 4500-7035 angstroms. Reducing of all obtained spectra was made.

The magnetic field of the gamma Cas star was measured using 144 pairs of spectra obtained on November 4, 2020 (73 spectra), January 2 and 3, 2021 (69 spectra), and May 30, 2021 (2 spectra). The rms magnetic fields obtained from all measurements made on these dates are 78+/-94 Gauss, 66+/-43 Gauss, 183+/-39 Gauss and 70+/-35 Gauss, respectively. The possible values of the local magnetic fields of gamma Cas are estimated. These values seems to be in the range from 50 G to 1 kG.

It has been established that at least five gamma Cas stars (HD 45314, HD 90563, HD 119682, NGC 6649-9, HD 110432) have states of low and high X-rays. In this case, the model characteristics of X-ray spectra (luminosity, hardness, temperature of the emitting plasma, contribution of the power-law component, spectral index, and others) change little. It is assumed that such changes in the X-ray spectra of gamma Cas type stars may be compelled with changes in the orientation of the decretion disks of stars or depend on the orbital phase of degenerate stellar satellites, if those are present.
A detailed analysis of the light curves of the bright gamma Cas star HD 110432 (BZ Cru), extracted from 6 archive X-ray observations made from 2002 to 2019, has been made. Three large X-ray quasi-flares (up to 20 kiloseconds) and several hundred short microflares were detected. The regular variations in the stellar X-rays with periods from 2 minutes to 6.5 hours and a fraction of periodic emission from 10 to 20% were discovered by the CLEAN Fourier analysis method. Some of these variations coincide with the periods of optical variations obtained by other authors. The X-ray spectra of the star were modeled during large quasi-flares and outside them. Significant changes in the model characteristics of these spectra were revealed. It is concluded that the most probable mechanism for the formation of X-ray emission of a gamma Cas star is the interaction of the magnetic field of the disk with the local magnetic fields of the star.

A catalog of all known gamma Cas stars and candidates to those is compiled. The catalog is available at http://www.astro.spbu.ru/LPV/?q=gcstars. The physical parameters of the stars listed in the catalog have been determined.

Results of the first year of the Project are presented on 5 oral and one invited reports at 5 conferences. 3 papers were published in peer-reviewed journals and an article in the proceedings of the All-Russian Conference “Solar and Solar-Terrestrial Physics-2023”.

Results of the second year (2024)

In 2024, the following scientific results were obtained:

Observations were carried out on the 6-m BTA telescope with the SCORPIO 1 spectrograph on June 24, 2024. 57 spectra of gamma Cas type stars SS 397 and 86 spectra of V2156 Cyg were obtained. Observations were carried out at the BTA on October 10-14, 2024 with the MSS spectrograph; 10 spectra of pi Aqr and 50 spectra of Be star ups Cyg, which is not part of the gamma Cas stars subgroup were registered.

In observations under the program of spectroscopic observations of Be stars at the Caucasus Mountain Observatory of the SAI MSU (KGO) Observations under the program of spectroscopic observations of Be stars at the KGO were carried out at the 2.5-meter SAI25 telescope of the KGO using the TDS spectrograph. In observations on January 18, 2024, 91 spectra of the gamma Cas type star V 558 Lyr in the blue and red channels were obtained. 745 spectra of the classical Be star omega Ori in the blue channel and 729 spectra of the star in the red channel were obtained at the KGO.

In result of the analysis of the gamma Cas type star SAO 49725 observations under the program of the project head on the 6-m BTA telescope with the SCORPIO 1 spectrograph in 2021; variability of the line profiles in its spectrum on short (70-223 minutes) scales was revealed. According to the photometric light curves of SAO 49725 obtained on the TESS satellite, regular brightness variations with a period of 1.1989+/-0.0012 days ( identified with the rotation period of the star) were detected. Analysis of spectral observations of the gamma Cas V 558 Lyr star at the 2.5-m KGO telescope showed the presence of regular variations in the H and He line profiles in the spectrum of the star with periods from 1 to 21 minutes. The component with a period of 1.22 minutes coincides, within the errors, with a period of 1.16 minutes of the X-ray brightness variations of the star. This component is possibly coincided with the rotation period of the white dwarf in the Be+WD V 558 Lyr binary system. An analysis of the rapid variability of line profiles in the spectrum of omega Ori obtained with the 2.5-m SAI25 telescope of the KGO was performed. Regular components of profile variations with periods from 37 to 150 minutes were found. Based on the photometric light curves obtained with the TESS satellite, regular brightness variations with a period of 0.94 days were detected, probably associated with non-radial pulsations of the star. Evidence of the possible accretion of the circumstellar decretion disk matter onto the star was found.

Archival observations of gamma Cas type stars obtained on the Chandra, XMM, and SRG satellites are analyzed. Regular components with periods close to the spectral lines variability periods in the X-ray light curves of these stars are found. Ultrafast variations in X-ray brightness with periods of 50–90 seconds are registered in gamma Cas type stars HD 45314, HD 45995, NGC6649 9, and V558 Lyr. These periods are possibly corresponded to the rotation periods of white dwarfs in the Be+WD binary systems

A method to detect microflares in X-ray light curves has been developed. The statistics of microflares detected in the light curves of 11 gamma Cas stars has been studied. From several dozen to several hundred microflares for each star at energies of 0.2-2 keV were detected. Most of the detected microflares have duration up to 5 s and a low relative power. The characteristics of X-ray microflares in the light curves of Be stars do not included in the gamma Cas type stars subgroup are studied. Approximately the same numbers of fast microflares occur in the X-ray radiation of these stars as on gamma Cas analog stars, but most of the flares occur at energies of up to 1 keV. This determines a smaller contribution of microflares to the X-ray luminosity of Be stars compared to gamma Cas stars.

A hybrid scenario of X-ray emission generation by gamma Cas type stars has been proposed. It is assumed that all stars of this type are wide binary systems consisting of a Be star and a white dwarf. The flare part of the total X-ray emission flux is formed by the interaction of the local magnetic fields of the star and the magnetic field of the disk, and the harder part is formed by the accretion of disk matter onto the rapidly rotating white dwarf.

The main results of the project are presented on the website "Superfast LPV of OBA stars" at an address http://www.astro.spbu.ru/LPV/. Based on the results of the research under the Project, 3 articles have been prepared and sent to publication in peer-reviewed scientific journals, two of which have already been published; one article has been accepted for publication. In addition, 4 articles based on the results of the Project have been accepted for publication in the proceedings of the All-Russian scientific conferences held in 2024.