# Be/X-ray binary

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**Be/X-ray binaries** (**BeXRB**s or **BeXB**s[1]) are a class of [high-mass X-ray binaries](/source/High-mass_X-ray_binaries) that consist of a [Be star](/source/Be_star) and a [neutron star](/source/Neutron_star). The neutron star is usually in a wide highly elliptical orbit around the Be star. The Be [stellar wind](/source/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 hot gas in a short time. As the gas falls onto the neutron star, a bright flare in hard [X-rays](/source/X-ray) is seen.[2]

## Definition and classification

Be/X-ray binaries belong to the high-mass X-ray binary category. The optical companion is a non-[supergiant](/source/Supergiant), fast-rotating Be type star with [emission lines](/source/Emission_line) indicating [luminosity class](/source/Luminosity_class) III-V. Most BeXRBs have eccentric [orbits](/source/Orbit) and contain a neutron star, confirmed through X-ray pulsations.[2]

BeXRBs are classified as either transient or persistent. Transient BeXRBs show two outburst types: type I outbursts are regular, periodic events occurring near periastron (the neutron star's closest orbital approach), while type II outbursts are major, unpredictable events reaching [Eddington luminosity](/source/Eddington_luminosity) () and lasting multiple orbital periods. Persistent BeXRBs display lower X-ray luminosity, more stable [light curves](/source/Light_curve) with minor variability, slower rotating neutron stars with [spin periods](/source/Spin_period) exceeding 200 seconds, and typically have wide, low-eccentricity orbits.[2]

## Physical properties

Be stars show emission [spectral lines](/source/Spectral_line) (particularly [Hα](/source/H-alpha)) and [infrared excess](/source/Infrared_excess) from their [circumstellar disk](/source/Circumstellar_disk). This disk forms from material expelled by the rapidly rotating Be star. Neutron stars in BeXRBs are also fast rotators, with spin periods ranging from a few seconds to several hundred seconds. These systems display a correlation between orbital and spin periods. Neutron stars usually exhibiting pulsations in their X-ray emission due to the strong magnetic field channeling accreted gas toward the magnetic poles.[2]

The neutron star truncates the Be star's disk through [tidal forces](/source/Tidal_force). Evidence for this includes correlation between Hα [equivalent width](/source/Equivalent_width) and orbital period, higher disk densities than in isolated Be stars, shorter V/R variability periods, and quantized [infrared](/source/Infrared) flux states matching resonant truncation radii. During [periastron](/source/Periastron), the neutron star accretes material from the disk, causing X-ray outbursts. The disk formation and dissipation timescales correlate with the system's orbital period.[2]

BeXRBs exist in both the [Milky Way](/source/Milky_Way) and [Magellanic Clouds](/source/Magellanic_Clouds), with the [Small Magellanic Cloud](/source/Small_Magellanic_Cloud) hosting many of these systems due to its lower [metallicity](/source/Metallicity) and higher [star formation](/source/Star_formation) rate.[2]

## Observations

Observationally, identification of BeXRBs relies heavily on their optical and IR signatures, primarily emission-line spectroscopy and IR photometry. A strong correlation exists between the intensity of IR colors and Hα emission lines, suggesting a common origin in the circumstellar disk. These diagnostics assist astronomers in identifying Be star candidates associated with newly discovered X-ray sources. Techniques such as photometric [color-color diagrams](/source/Color-color_diagram) and narrow-slit [spectroscopy](/source/Spectroscopy) are used to accurately identify these optical counterparts.[2]

## Notable BeXRBs

### X Persei

Main article: [X Persei](/source/X_Persei)

X Persei is a binary system containing a [γ Cassiopeiae variable](/source/%CE%93_Cassiopeiae_variable) and a [pulsar](/source/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.[3]

### LSI+61°303

Main article: [LS I +61 303](/source/LS_I_%2B61_303)

LSI+61°303 is a possible example of a BeXRB. 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.[4] 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 a [black hole](/source/Black_hole).[5]

Of the 20 or so members of the Be/X-ray binary class, as of 1996, only [X Persei](/source/X_Persei) and LSI+61°303 have X-ray outbursts of much higher luminosity and harder spectrum (*[kT](/source/KT_(energy))* ≈ 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](/source/SS_433), [Cyg X-3](/source/Cygnus_X-3) and [Cir X-1](/source/Circinus_X-1).[4]

### RX J0209.6-7427

RX J0209.6-7427 is a BeXRB located in the [Magellanic Bridge](/source/Magellanic_Bridge).[6] 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 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](/source/Magellanic_Clouds).[7][8][9]

### Swift J010902.6-723710

Swift J010902.6-723710 is a BeXRB detected by the Swift Small Magellanic Cloud (SMC) Survey (S-CUBED). An X-ray outburst, detected on October 10, 2023, had characteristics of Type I and II outbursts. Proposed orbital period is 60.623 days. Companion star of the system is "B0-0.5 star of spectral class Ve". The system's neutron star has large accretion disk.[10][11]

## References

1. Zhou, Yungang; Wang, Dehua; Zhang, Chengmin (February 6, 2025). "From Be X-Ray Binaries to Double Neutron Stars: Exploring the Spin and Orbital Evolution". *Universe*. **11** (2): 51. [Bibcode:2025Univ...11...51Z](https://ui.adsabs.harvard.edu/abs/2025Univ...11...51Z). [doi:10.3390/universe11020051](https://doi.org/10.3390/universe11020051)

1. Reig, Pablo (2011). "Be/X-ray binaries". *Astrophysics and Space Science*. **332** (1): 1–29. [arXiv:1101.5036](https://arxiv.org/abs/1101.5036). [Bibcode:2011Ap&SS.332....1R](https://ui.adsabs.harvard.edu/abs/2011Ap%26SS.332....1R). [doi:10.1007/s10509-010-0575-8](https://doi.org/10.1007/s10509-010-0575-8)

1. 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](https://arxiv.org/abs/1408.3542). [Bibcode:2014AJ....148..113L](https://ui.adsabs.harvard.edu/abs/2014AJ....148..113L). [doi:10.1088/0004-6256/148/6/113](https://doi.org/10.1088/0004-6256/148/6/113)

1. 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](https://ui.adsabs.harvard.edu/abs/1996A%26A...305..817T)

1. 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](https://arxiv.org/abs/1704.01335). [Bibcode:2017MNRAS.468.3689M](https://ui.adsabs.harvard.edu/abs/2017MNRAS.468.3689M). [doi:10.1093/mnras/stx778](https://doi.org/10.1093/mnras/stx778)

1. 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](https://ui.adsabs.harvard.edu/abs/2005A%26A...435....9K). [doi:10.1051/0004-6361:20042408](https://doi.org/10.1051/0004-6361:20042408)

1. 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](https://arxiv.org/abs/2004.04930). [Bibcode:2020MNRAS.495.2664C](https://ui.adsabs.harvard.edu/abs/2020MNRAS.495.2664C). [doi:10.1093/mnras/staa1041](https://doi.org/10.1093/mnras/staa1041)

1. ["Ultra-bright X-ray source awakens near a galaxy not so far away"](https://ras.ac.uk/news-and-press/research-highlights/ultra-bright-x-ray-source-awakens-near-galaxy-not-so-far-away). *Royal Astronomical Society*. June 2020.

1. ["Ultra-Bright Pulsar Awakens Next Door To The Milky Way After 26-Year Slumber"](https://www.iflscience.com/space/ultrabright-pulsar-awakens-next-door-to-the-milky-way-after-26year-slumber/). *Alfredo Carpineti*. June 2020.

1. Nowakowski, Tomasz. ["Astronomers discover a rare eclipsing X-ray binary"](https://phys.org/news/2024-03-astronomers-rare-eclipsing-ray-binary.html). *phys.org*. Retrieved 28 March 2024.

1. Gaudin, Thomas M.; Kennea, Jamie A.; Coe, Malcolm J.; Monageng, Itumeleng M.; Udalski, Andrzej; Townsend, Lee J.; Buckley, David A. H.; Evans, Phil A. (2024). "Discovery of a Rare Eclipsing Be/X-ray Binary System, Swift J010902.6-723710 = SXP 182". *The Astrophysical Journal*. **965** (1): L10. [arXiv:2403.05648](https://arxiv.org/abs/2403.05648). [Bibcode:2024ApJ...965L..10G](https://ui.adsabs.harvard.edu/abs/2024ApJ...965L..10G). [doi:10.3847/2041-8213/ad354a](https://doi.org/10.3847/2041-8213/ad354a)

## Further reading

- Liu, Boyuan; Sartorio, Nina S; Izzard, Robert G; Fialkov, Anastasia (27 November 2023). "Population synthesis of Be X-ray binaries: metallicity dependence of total X-ray outputs". *Monthly Notices of the Royal Astronomical Society*. **527** (3): 5023–5048. [arXiv:2308.06154](https://arxiv.org/abs/2308.06154). [doi:10.1093/mnras/stad3475](https://doi.org/10.1093/mnras/stad3475)
- Coe, M. J. (2000). ["Be stars in X-ray binary systems"](https://adsabs.harvard.edu/full/2000ASPC..214..656C). *The be Phenomenon in Early-Type Stars, IAU Colloquium 175, ASP Conference Proceedings*. **214**: 656. Astronomical Society of the Pacific. [arXiv:astro-ph/9911272](https://arxiv.org/abs/astro-ph/9911272). [Bibcode:2000ASPC..214..656C](https://ui.adsabs.harvard.edu/abs/2000ASPC..214..656C). ISBN 1-58381-045-5.

## External links

- [Be/X-ray binaries and candidates](http://xray.sai.msu.ru/~raguzova/BeXcat/) by Natalya V. Raguzova and Sergei B. Popov

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Adapted from the Wikipedia article [Be/X-ray binary](https://en.wikipedia.org/wiki/Be%2FX-ray_binary) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Be%2FX-ray_binary?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.