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

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

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]

RX J0209.6-7427

RX J0209.6-7427 is a Be/X-ray binary star located in the Magellanic Bridge.[5] 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.[6][7][8]

References

  1. ↑ 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.
  2. ↑ 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.
  3. 1 2 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.
  4. ↑ 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.
  5. ↑ Kahabka, P.; Hilker, M. (2005). "Discovery of an X-ray binary in the outer SMC wing". Astronomy and Astrophysics. 435 (1): 9–16. Bibcode:2005A&A...435....9K. doi:10.1051/0004-6361:20042408.
  6. ↑ Chandra, A. D.; Roy, J.; Agrawal, P. C.; Choudhury, M. (2020). "Study of recent outburst in the Be/X-ray binary RX J0209.6−7427 with AstroSat: a new ultraluminous X-ray pulsar in the Magellanic Bridge?". Monthly Notices of the Royal Astronomical Society. 495 (3): 2664–2672. arXiv:2004.04930. Bibcode:2020MNRAS.495.2664C. doi:10.1093/mnras/staa1041.
  7. ↑ "Ultra-bright X-ray source awakens near a galaxy not so far away". Royal Astronomical Society. June 2020.
  8. ↑ "Ultra-Bright Pulsar Awakens Next Door To The Milky Way After 26-Year Slumber". Alfredo Carpineti. June 2020.


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