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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.
LSI+61°303 is an 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. Using the ROSAT and VLA observatories, an X-ray outburst with an ~10 d duration was detected. LSI+61°303 is a variable radio source characterized by periodic, non-thermal radio outbursts with a period of 26.496 d. The 26.5 d period has been attributed to the eccentric orbital motion of a compact object, probably a neutron star, around a rapidly rotating B0 Ve star, with a Teff ~26,000 K and luminosity of 1031 watts (1038 erg s−1). Photometric observations at optical and infrared wavelengths also show a 26.5 d modulation.
Of the 20 or so members of the Be/X-ray binary systems, as of 1996, only X Per and LSI+61°303 have X-ray outbursts of much higher luminosity and harder spectrum (kT ≈ 10–20 keV) vs. (kT ≤ 1 keV); however, LSI+61°303 further distinguishes itself by its strong, outbursting radio emission. "The radio properties of LSI+61°303 are similar to those of the "standard" high-mass X-ray binaries such as SS 433, Cyg X-3 and Cir X-1."
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