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Be/X-ray binaries (BeXRBs) are a class of high-mass X-ray binaries that consist of a Be star and a neutron star. 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-rays is seen.[1]
X Persei
Main article: X Persei

X Persei is a binary system containing a γ Cassiopeiae variable 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.[2]
LSI+61°303
Main article: LS I +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.[3] 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.[4]

Of the 20 or so members of the Be/X-ray binary class, as of 1996, only X Persei 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.[3]
References

Reig, Pablo (2011). "Be/X-ray binaries". Astrophysics and Space Science. 332: 1. arXiv:1101.5036. Bibcode:2011Ap&SS.332....1R. doi:10.1007/s10509-010-0575-8.
Li, Hui; Yan, Jingzhi; Zhou, Jianeng; Liu, Qingzhong (2014). "Long-term Optical Observations of the Be/X-Ray Binary X Per". The Astronomical Journal. 148 (6): 113. arXiv:1408.3542. Bibcode:2014AJ....148..113L. doi:10.1088/0004-6256/148/6/113.
Taylor AR, Young G, Peracaula M, Kenny HT, Gregory PC (1996). "An X-ray outburst from the radio emitting X-ray binary LSI+61°303". Astron. Astrophys. 305: 817–24. Bibcode:1996A&A...305..817T.

Massi, M; Migliari, S; Chernyakova, M (2017). "The black hole candidate LS I +61°0303". Monthly Notices of the Royal Astronomical Society. 468 (3): 3689. arXiv:1704.01335. Bibcode:2017MNRAS.468.3689M. doi:10.1093/mnras/stx778.

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