On August 11, 1935,R. F. Sanford found a weakemission line ofHydrogen–β in thespectrum of this star, and it was included in the 1943 supplement to theMount Wilson catalogue of similar stars with the identifier MWC 656.[11] In 1964, it was assigned astellar classification of B5:ne, where 'B5' indicates this is aB-type star, 'n' means it displays 'nebular' lines due to rapid rotation, and 'e' shows it has emission lines. The ':' suffix indicates some uncertainty about the classification.[12] It was included in a catalogue ofBe stars in 1982.[13] In 2005 it was found to have highprojected rotational velocity of305 km/s.[8]
A light curve for MWC 656. The blue points show theHipparcos data.[14] The red curve shows asine function fit to that data, and the green points show the averages of the data within 10 bins across each period.
In 2009, theAGILE satellite discovered a nearby source ofgamma-ray emission above100 MeV. This source was given the identifier AGL J2241+4454. HD 215227 is the only suitable optical counterpart to lay within the 0.6° error circle. Spectra from the star showed evidence of emission from acircumstellar disk, as well as absorption from ashell feature. Rapid changes in emission line variability suggest an orbiting companion that is tidally interacting with the disk.Hipparcoslight curve data indicated anorbital period of60.37±0.04 d.[10] This was confirmed in 2012 via radial velocity measurements of helium lines in photosphere of the Be star.[15]
Refined radial velocity measurements in 2014 indicated a massive companion in the range of3.8–6.9 M☉, assuming the Be star has a mass of10–16 M☉. Amain sequence companion with a mass this high should be readily visible in the optical band. Likewise, asubdwarf or astripped helium core from a massive progenitor star don't fit the observations. The mass is too high for awhite dwarf or aneutron star, leaving astellar mass black hole as the only viable candidate.[16][17] A faintX-ray emission was detected later the same year with a total luminosity of(3.7±1.7)×1031 erg·s−1, making this a high massX-ray binary system. This luminosity is consistent with a stellar black hole in quiescence – meaning very little material is being fed into the black hole from the primary star.[18]
This was the first reported binary system combining a black hole with a Be star.[6] However, many Be stars are now found to have subdwarfOC companions, and the properties of these appear similar to MWC 656. The 2022 discovery oftidal distortion of the disk orbiting the Be star invalidated the original radial velocity amplitude, which called into question the 2014 mass estimates. The correction for this probably rules out a black hole companion. Emission from ionized helium near the companion appears double-peaked, indicating there is an orbiting accretion disk being fed from the disk orbiting the Be star.[19] Revised measurements reported in 2023 found a mass range of0.94 to 2.4 M☉ for the companion, which means this is instead aneutron star, awhite dwarf, or a hot helium star.[5]
The position of the star at a distance of 2,300 light-years (0.71 kpc) below thegalactic plane suggests this is arunaway star system, since it is a young star not located near anystar forming region.[10] This scenario favors the neutron star companion.[5]
^abChen, W. P.; Lee, H. T. (December 2008), Reipurth, Bo (ed.), "The Lacerta OB1 Association",Handbook of Star Forming Regions, Volume I: The Northern Sky, ASP Monograph Publications, vol. 4, p. 124,arXiv:0811.0443,Bibcode:2008hsf1.book..124C.
^abGlebocki, R.; Gnacinski, P. (2005), "Catalog of Stellar Rotational Velocities",VizieR On-line Data Catalog: III/244. Originally Published in: 2005csss...13..571G; 2005yCat.3244....0G,3244,Bibcode:2005yCat.3244....0G.
^Merrill, Paul W.;Burwell, Cora G. (1943), "No. 682. Supplement to the Mount Wilson catalogue and bibliography of stars of classes B and A whose spectra have bright hydrogen lines",Contributions from the Mount Wilson Observatory,682, Carnegie Institution of Washington:1–32,Bibcode:1943CMWCI.682....1M.
^Jaschek, Carlos; et al. (1964), "Catalogue of stellar spectra classified in the Morgan-Keenan system",Observatorio Astronomico de la Universidad de la Plata, Serie Astronomica, La Plata, vol. 28, p. 1,Bibcode:1964PLPla..28....1J.
^Jaschek, M.; Egret, D. (April 1982), "Catalog of Be stars",IAU Symposium, vol. 98, p. 261,Bibcode:1982IAUS...98..261J.
Zamanov, Radoslav K.; et al. (July 2022), "Optical spectroscopy of the Be/black hole binary MWC 656 - interaction of a black hole with a circumstellar disc",Astronomische Nachrichten,343 (6–7), id. e24019,arXiv:2206.06013,Bibcode:2022AN....34324019Z,doi:10.1002/asna.20224019.
Grudzinska, M.; et al. (September 2015), "On the formation and evolution of the first Be star in a black hole binary MWC 656",Monthly Notices of the Royal Astronomical Society,452 (3):2773–2787,arXiv:1504.03146,Bibcode:2015MNRAS.452.2773G,doi:10.1093/mnras/stv1419.
Alexander, Michael J.; McSwain, M. Virginia (May 2015), "An updated gamma-ray analysis of the Be-BH binary HD 215227",Monthly Notices of the Royal Astronomical Society,449 (2):1686–1690,arXiv:1502.07385,Bibcode:2015MNRAS.449.1686A,doi:10.1093/mnras/stv400.
