[Paper Review] MWC656: A Be+BH or a Be+sdO?

Th. Rivinius, R. Klement+4 more|arXiv (Cornell University)|Aug 25, 2022

Astrophysical Phenomena and Observations4 citations

TL;DR

This study re-evaluates MWC 656, originally proposed as a Be+black hole binary, by analyzing high-resolution spectroscopy and polarimetry. It finds that radial velocity variations in photospheric lines and emission line behavior are inconsistent with a black hole, instead supporting a Be+hot subdwarf (sdO) binary system with a companion mass far below the black hole threshold.

ABSTRACT

MWC656 has been reported as classical Be star with a black hole companion. Revisited spectral variability properties render this unlikely, with a hot subdwarf more probable.

Motivation & Objective

• Reassess the nature of MWC 656, initially classified as a Be+black hole binary, due to conflicting spectral variability and orbital parameters.
• Address the inconsistency in radial velocity measurements used to infer a black hole companion, particularly the reliance on double-peaked emission lines that are not reliable for orbital motion.
• Investigate whether the system's spectral behavior aligns more closely with known Be+sdO binaries, which exhibit similar emission and absorption features.
• Re-analyze the orbital period and radial velocity amplitudes using high-resolution spectra and polarimetry to reassess the companion mass.
• Determine whether the lack of X-ray emission supports a quiescent black hole or instead indicates a non-accreting hot subdwarf.

Proposed method

• Re-analyzed original spectroscopic data (R ≈ 5500) from Casares et al. (2014) and combined it with new high-resolution spectra from ARCES (R = 31,500) and ESPaDOnS (R = 48,000).
• Performed Fourier analysis on radial velocity measurements of He ii λ4686 to determine a revised orbital period of 59.12 ± 0.05 days.
• Used Gaussian fitting to measure radial velocity amplitudes of He ii λ4686 emission, which traces the companion’s motion, yielding K₂ ≈ 80 km/s.
• Identified and measured photospheric He i λ6678 lines to estimate the true radial velocity amplitude K₁ of the Be star, finding it to be 10–15 km/s.
• Analyzed the phase-locked absorption component in He i λ6678, which appears as a deep shell absorption at phase ~0.4, indicating orbital modulation by a compact companion.
• Evaluated the variability of double-peaked emission lines (e.g., Fe ii λ5317) and their peak separation changes due to tidal disk distortion, which invalidates their use for measuring K₁.

Experimental results

Research questions

• RQ1 Is the orbital period of MWC 656 consistent with the 60.37-day value derived from photometric data, or is it better constrained by spectroscopic data?
• RQ2 Can the radial velocity amplitude of the Be star (K₁) be reliably measured from double-peaked emission lines, or are they distorted by tidal effects in the disk?
• RQ3 What is the true radial velocity amplitude of the Be star based on photospheric lines, and how does it affect the inferred companion mass?
• RQ4 Do the observed emission and absorption features in the spectra match those of known Be+sdO binaries, such as φ Per or HD 55606?
• RQ5 Does the absence of X-ray emission support a quiescent black hole, or is it more consistent with a non-accreting hot subdwarf?

Key findings

• The revised orbital period of MWC 656 is 59.12 ± 0.05 days, derived from Fourier analysis of high-resolution He ii λ4686 radial velocities.
• The radial velocity amplitude of the Be star (K₁) is measured at 10–15 km/s from photospheric He i λ6678 lines, significantly lower than the 32 km/s reported by Casares et al. (2014).
• The H α bisector radial velocity amplitude is even lower at ~7 km/s, further reducing the inferred companion mass.
• The double-peaked emission lines (e.g., Fe ii λ5317) exhibit variable peak separation due to tidal distortion of the Be star’s disk, making them unsuitable for measuring K₁.
• A deep, phase-locked absorption component in He i λ6678 at phase ~0.4 is consistent with gas streams from a hot subdwarf companion, as seen in other Be+sdO systems.
• The lack of X-ray emission does not support a quiescent black hole, as the spectroscopic mapping reveals ongoing gas dynamics inconsistent with a non-accreting black hole.