We present new spectroscopic observations of the Be/black hole binary MWC 656 obtained during the period 2015–2021. We measure the equivalent width of Hα (EWα), Hβ (EWβ$$ {EW}_{\beta } $$), and the distance between the peaks of Hα (normalΔVα$$ \Delta {V}_{\alpha } $$), Hβ (normalΔVβ$$ \Delta {V}_{\beta } $$), and FeII (normalΔVFeII$$ \Delta {V}_{\mathrm{FeII}} $$) lines. Combining new and old data, we find that: (1) the density of the circumstellar disc of MWC 656 (normalΔVα$$ \Delta {V}_{\alpha } $$ versus EWα$$ {EW}_{\alpha } $$ diagram) is similar to the Be stars. For the Be stars, we find the relation normalΔVβ=0.999normalΔVα+62.4$$ \Delta {V}_{\beta }=0.999\Delta {V}_{\alpha }+62.4 $$ km s−1, and the position of MWC 656 corresponds to the average behavior of the Be stars. This means that the presence of the black hole does not change the overall structure of the circumstellar disc; (2) the periodogram analysis indicates modulation of EWα$$ {EW}_{\alpha } $$ with a period 60.4±0.4$$ 60.4\pm 0.4 $$ days, which is identical to the binary orbital period. The maxima of EWα$$ {EW}_{\alpha } $$ and EWβ$$ {EW}_{\beta } $$ are around periastron (phase zero); (3) around orbital phase zero, normalΔVβ$$ \Delta {V}_{\beta } $$ and normalΔVFeII$$ \Delta {V}_{FeII} $$ decrease by about 30$$ 30 $$ km s−1. This suggests that we observe an increase of the circumstellar disc size induced by the periastron passage of the black hole and that the entire circumstellar disc pulsates with the orbital period with relative amplitude of 10–20%. The observations also indicate, that the reason for the black hole in MWC 656 to be in deep quiescence is a very low efficiency of accretion (∼2×10−6$$ \sim 2\times {10}^{-6} $$).
In this work, we search for negative superhumps (nSHs) in poorly studied cataclysmic variables using TESS data. We find three eclipsing binaries with nSH signatures: HBHA 4204-09, Gaia DR3 5931071148325476992, and SDSS J090113.51+144704.6. The last one exhibits IW And-like behaviour in archival ZTF data, and appears to have shallow, grazing eclipses. In addition, we detect nSH signatures in two non-eclipsing systems: KQ Mon and Gaia DR3 4684361817175293440, by identifying the orbital period from the superorbital-dependent irradiation of the secondary. We discover nSH signatures in one more system, [PK2008] HalphaJ103959, by using an orbital period from another work. An improved mass ratio – nSH deficit relation q(ϵ−) is suggested by us, which agrees with independent measurements on nova-like variables. With this relation, we estimate the mass ratios of all systems in our sample, and determine the orbital inclinations for the three that are eclipsing. All systems with discovered nSHs in this work are excellent targets for follow-up spectroscopic studies.
The Be stars display variable optical emission lines originating in the circumstellar disc. Here we analyze high resolution spectroscopic observations of Be stars and the distance between the peaks of H𝛼, H𝛽, and H𝛾 emission lines (ΔV 𝛼 ,ΔV 𝛽 , and ΔV 𝛾 respectively). Combining published data, spectra from the ELODIE archive (obtained in the period 1998-2003) and Rozhen spectra (obtained 2015-2023) of 93 Be stars, we find a set of relations connecting ΔV 𝛼 , ΔV 𝛽 and ΔV 𝛾 . They are effective for 30 ≤ ΔV 𝛼 ≤ 500 km s −1 , 80 ≤ ΔV 𝛽 ≤ 600 km s −1 , and 40 ≤ ΔV 𝛾 ≤ 300 km s −1 . The new equations are in the form y = ax + b and are valid for a wider velocity range than in previous studies.
We present a detailed photometric study of the bright cataclysmic variable BG Tri using ground-based observations mainly from Rozhen Observatory, ASAS-SN, TESS, and WASP sky surveys. We report the discovery of a negative superhump with P−sh = 0.1515(2) days and a co-existing superorbital variation with P = 3.94(53) days in data from 2019 and 2020. A positive superhump with P+sh = 0.1727(14) days is also discovered in data from 2006. The obtained negative superhump deficit ϵ− = 0.044(1) and the positive superhump excess ϵ+ = 0.090(9) give us an independent photometric evaluation of the mass ratio (q) of the system, which we find to be q− = 0.37(2) and q+ = 0.40(5) respectively. We also present a study of the quasi-periodic oscillations (QPOs) and stochastic variability (flickering) in BG Tri. The light curves show a rich mixture of simultaneously overlapping quasi-periods ranging from 5 to 25 minutes. The multi-colour (UBVRI) photometric observations from Rozhen Observatory reveal the typical increase of the flickering amplitudes to the shorter wavelengths. The recently introduced A60 amplitude of the flickering light source in all studied photometric bands is systematically lower when the negative superhump is gone in season 2021.
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