Be/X-ray binaries (BeXRBs) are a class of

high-mass X-ray binaries 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 normal star), to the other component, called the ''accretor'', whi ...

that consist of a

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 e ...

and a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

. The neutron star is usually in a wide highly elliptical orbit around the Be star. The Be stellar wind forms a disk confined to a plane often different from the orbital plane of the neutron star. When the neutron star passes through the Be disk, it accretes a large mass of gas in a short time. As the gas falls onto the neutron star, a bright flare in hard

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s is seen.

X Persei

X Persei is a binary system containing a

γ Cassiopeiae variable A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The bro ...

and a pulsar. It has a relatively long period and low eccentricity for this type of binary, which means the x-ray emission is persistent and not usually strongly variable. Some strong x-ray flares have been observed, presumably related to changes in the accretion disc, but no correlations have been found with the strong optical variations.

LSI+61°303

LSI+61°303 is a possible example of a Be/X-ray binary star. It is a periodic, radio-emitting binary system that is also the gamma-ray source, CG135+01. It is also a variable radio source characterized by periodic, non-thermal radio outbursts with a period of 26.496 d. The 26.5 d period is attributed to the eccentric orbital motion of a compact object, possibly a neutron star, around a rapidly rotating B0 Ve star. Photometric observations at optical and infrared wavelengths also show a 26.5 d modulation. Although the mass of the compact object in the LS I +61 303 system is not known accurately, it is likely that it is too large to be a neutron star and so it is likely to be a black hole. Of the 20 or so members of the Be/X-ray binary class, as of 1996, only

X Persei X Persei is a high-mass X-ray binary system located in the constellation Perseus, approximately 950 parsecs away. It is catalogued as 4U 0352+309 in the final Uhuru catalog of X-ray objects. The conventional star component of X Pers ...

and LSI+61°303 have X-ray outbursts of much higher luminosity and harder spectrum ('' kT'' ≈ 10–20 keV) vs. (''kT'' ≤ 1 keV). LSI+61°303 also shows strong radio outbursts, more similar to those of the "standard" short-period high-mass X-ray binaries such as SS 433, Cyg X-3 and Cir X-1.

RX J0209.6-7427

RX J0209.6-7427 is a Be/X-ray binary star located in the Magellanic Bridge. A couple of rare outbursts have been observed from this source hosting a neutron star. The last outburst was detected in 2019 after about 26 years. The accreting neutron star in this Be/X-ray binary system is an ultraluminous X-ray Pulsar (ULXP) making it the second closest ULXP and the first ULXP in our neighbouring Galaxy 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 bo ...

References

*Be X-ray binary *Be X-ray binary {{star-system-stub