Optical spectroscopy of the Be/black hole binary MWC656 -- interaction of a black hole with a circumstellar disc

Abstract

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 H$\alpha$ (EW${}_{\alpha }$), H$\beta$ ($E{W}_{\beta }$), and the distance between the peaks of H$\alpha$ ($\Delta V_{\alpha }$), H$\beta$ ($\Delta V_{\beta }$), and FeII ($\Delta V_{FeII}$) lines. Combining new and old data, we find that: - the density of the circumstellar disc of MWC 656 ($\Delta V_{\alpha }$ versus $E{W}_{\alpha }$ diagram) is similar to the Be stars. For the Be stars we find the relation $\Delta V_{\beta }=0.999\Delta V_{\alpha }+62.4$ km s${}^{-1}$, and the position of MWC~656 corresponds to the average behaviour of the Be stars. This means that the presence of the black hole does not change the overall structure of the circumstellar disc. - the periodogram analysis indicates modulation of $E{W}_{\alpha }$ with a period $60.4\pm 0.4$ days, which is identical to the binary orbital period. The maxima of $E{W}_{\alpha }$ and $E{W}_{\beta }$ are around periastron (phase zero). - around orbital phase zero, $\Delta V_{\beta }$ and $\Delta V_{FeII}$ decrease by about $30$ 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 ($\sim 2\times 10^{-6}$).

Accepted by Astronomische Nachrichten / Astronomical Notes

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Cited by in corpus (2)

• MWC 656 is unlikely to contain a black hole
• Optical spectroscopy of Be stars: peak separation of Balmer emission lines