Be–white dwarf X-ray binary systems (BeWDs) are a rare type of

X-ray binary X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively common main sequence star), to the other component, called the ''acc ...

consisting of a

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

that accretes matter from a rapidly-rotating

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer ...

. These systems form through binary evolution where mass transfer spins up the accretor to become a Be star while the donor evolves into a white dwarf. BeWDs probably originate from a Be star and a subdwarf O or B star binaries. Population synthesis models indicate these systems can evolve through two primary pathways: * Approximately 60-70% merge into red giants that observationally look like

luminous red nova A luminous red nova (abbr. ''LRN'', pl. ''luminous red novae'', pl.abbr. ''LRNe'') is a stellar explosion thought to be caused by the merging of two stars. They are characterised by a distinct red colour, and a light curve that fades slowly with ...

e. * About 30-40% evolve into double white dwarf systems that may be detectable as gravitational wave sources by

Laser Interferometer Space Antenna The Laser Interferometer Space Antenna (LISA) is a planned space probe to detect and measure gravitational waves—tiny ripples in the fabric of spacetime—from astronomical sources. LISA will be the first dedicated space-based gravitational-w ...

(LISA), and will be its "most likely gravitational wave source". The formation requires specific initial conditions: the primary must transfer sufficient mass to spin up the secondary to Be star velocities without triggering common envelope evolution. Tidal synchronization mechanisms explain the observed lack of BeWDs with orbital periods shorter than 17 days. BeWDs can be identified by several features: * Supersoft X-ray emission (kT ~ 0.1 keV) * X-ray luminosities of 10³³-10³⁸ erg/s * Deep

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

(0.67 keV) and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(0.87 keV) absorption edges in X-ray spectra The white dwarfs in these systems tend to be massive (0.9-1.35 ) with surface temperatures of 20,000-40,000 K. Detection is challenging as the white dwarf is often embedded within the Be star's decretion disk, absorbing most extreme-UV and soft X-ray photons. Some studies suggest that γ Cas stars, a subgroup of Be stars exhibiting bright X-ray emission, likely have white dwarf companions rather than hot subdwarf stars or main sequence stars, as interferometric observations show no detectable companion flux while the systems' properties match theoretical predictions for Be+WD binaries. Despite theoretical predictions that BeWDs should be 7 times more common than Be-neutron star systems, only 8 have been confirmed as of 2025. According to different numerical models, 40 to 80% of Be stars should have white dwarf companions. * 46% per * 70% per * 70-80% per All identified systems are located in the

Magellanic Clouds The Magellanic Clouds (''Magellanic system'' or ''Nubeculae Magellani'') are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both ...

rather than the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, possibly due to lower extinction rates allowing easier detection of soft X-rays, or because of the different

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

of the Magellanic Clouds which may be related to the formation of BeWDs.

References

{{reflist White dwarfs Be stars X-ray binaries