On the nature and Galactic origin of the Be binary MWC 656. New insights from VLA, Gaia, and Fermi-LAT
Sergio A. Dzib, Frederic Jaron
TL;DR
MWC 656, a Be-star binary once proposed as Be+BH, is re-examined using archival VLA radio data, Gaia DR3 astrometry, and Fermi-LAT gamma-ray observations. A deeper VLA X-band image yields a robust radio detection with $S_\nu=4.6\pm0.8\,\mu$Jy and $\alpha=1.2\pm1.8$, placing the radio/X-ray relation in a regime compatible with both the BH fundamental plane and Güdel-Benz relation, leaving the emission mechanism unresolved. Gaia-based kinematics show a small peculiar velocity $v_{pec}=11.2\pm2.3$ km s$^{-1}$ and a height of $442$ pc below the plane, with a travel time incompatible with a plane-origin runaway and consistent with in-situ formation at high latitude; the orbit also aligns with Galactic rotation, reducing deceleration likelihood. Fermi-LAT data give no significant $\gamma$-ray detection over 17.1 years, including orbital-phase-resolved searches. Collectively, the evidence weakens the Be+BH interpretation and favors a non-BH compact object (white dwarf or neutron star) as the companion, though the high-latitude formation remains intriguing.
Abstract
The binary star MWC 656 was initially proposed as the first confirmed system composed of a Be star and a black hole. However, recent studies have challenged this interpretation, suggesting that the compact companion is unlikely to be a black hole. In this study, we revisit the nature of MWC 656 by analyzing archival data across multiple wavelengths, including radio observations from the VLA, optical astrometry from the Gaia satellite, and high-energy $γ$-ray data from the Fermi-LAT. Using all available VLA observations at X-band (8.0-12.0 GHz), we produce the deepest radio map toward this system to date, with a noise level of 780 nJy beam$^{-1}$. The source MWC 656 is detected with $S_ν=4.6\pm0.8μ$Jy and a spectral index of $α=1.2\pm1.8$, derived by sub-band imaging. The radio and X-ray luminosity ratio of MWC 656 is consistent with both the fundamental plane of accreting black holes and with the Güdel-Benz relation for magnetically active stars, leaving the emission mechanism ambiguous. The optical astrometric results of MWC 656 indicate a peculiar velocity of $11.2\pm2.3$ km s$^{-1}$, discarding it as a runaway star. Its current location, 442 pc below the Galactic plane, implies a vertical travel time incompatible with the lifetime of a B1.5-type star. Moreover, the agreement between observed and expected motion in all three velocity components argues against a deceleration scenario, suggesting that MWC 656 likely formed in situ at high Galactic latitude. We carried out maximum likelihood analysis of Fermi-LAT data, but cannot report a significant detection of $γ$-ray emission from this source. These results reinforce recent evidence that challenge the black hole companion interpretation, and favor a non-BH compact object such as a white dwarf or neutron star.