MWC 656

Lore & Background

MWC 656 was first noted on August 11, 1935, when R. F. Sanford observed a weak emission line of Hydrogen–β in its spectrum, leading to its inclusion in the 1943 supplement to the Mount Wilson catalogue as MWC 656. In 1964 it received a stellar classification of B5:ne, indicating a B-type star with rapid rotation and emission lines, though with some uncertainty. It was cataloged among Be stars in 1982, and in 2005 its high projected rotational velocity of 305 km/s was measured.

In 2009, the AGILE satellite detected a gamma-ray source above 100 MeV, designated AGL J2241+4454, with HD 215227 as the only suitable optical counterpart within the 0.6° error circle. Spectra revealed a circumstellar disk and shell absorption, and rapid emission line variability suggested an orbiting companion. Hipparcos light curve data indicated an orbital period of 60.37±0.04 days, confirmed in 2012 by radial velocity measurements of helium lines.

Refined radial velocity measurements in 2014 pointed to a massive companion of 3.8–6.9 M☉, too high for a white dwarf or neutron star, leaving a stellar-mass black hole as the only viable candidate. Faint X-ray emission detected later that year was consistent with a quiescent black hole. However, the 2022 discovery of tidal distortion of the disk orbiting the Be star invalidated the original radial velocity amplitude, and revised 2023 measurements gave a companion mass of 0.94–2.4 M☉, suggesting a neutron star, white dwarf, or hot helium star instead. The system's position 2,300 light-years below the galactic plane indicates it is a runaway star system, favoring the neutron star scenario.

Reader's Guide

MWC 656 is significant as a high-mass X-ray binary that was initially identified as the first known system pairing a Be star with a black hole. This claim, based on 2014 radial velocity data, attracted considerable attention because such binaries are rare and important for understanding stellar evolution and compact object formation. However, subsequent observations in 2022 and 2023 revealed that tidal distortion of the Be star's disk had skewed the earlier measurements, and the corrected mass range of 0.94–2.4 M☉ for the companion rules out a black hole, instead pointing to a neutron star, white dwarf, or hot helium star. The system's status as a runaway star, far from any star-forming region, adds to its interest. The case of MWC 656 illustrates the challenges in accurately determining companion masses in binary systems and the importance of accounting for disk interactions. It remains a valuable object for studying Be stars and their compact companions, though its exact nature is still debated.

Did You Know?

• MWC 656 was first noted in 1935 by R. F. Sanford for a weak hydrogen emission line.
• It was the first reported binary system combining a black hole with a Be star, though later measurements cast doubt on that classification.
• The system lies about 2,300 light-years below the galactic plane, suggesting it is a runaway star system.
• Revised 2023 measurements gave the companion a mass range of 0.94 to 2.4 M☉, ruling out a black hole.