Editing Be/X-ray binary

'''Be/X-ray binaries''' ('''BeXRB'''s or '''BeXB'''s<ref>{{Cite journal|title=From Be X-Ray Binaries to Double Neutron Stars: Exploring the Spin and Orbital Evolution|first1=Yungang|last1=Zhou|first2=Dehua|last2=Wang|first3=Chengmin|last3=Zhang|date=February 6, 2025|journal=Universe|volume=11|issue=2|pages=51|doi=10.3390/universe11020051|doi-access=free |bibcode=2025Univ...11...51Z }}</ref>) 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 hot gas in a short time. As the gas falls onto the neutron star, a bright flare in hard [[X-ray]]s is seen.<ref name=reig/>

== Definition and classification ==

Be/X-ray binaries belong to the high-mass X-ray binary category. The optical companion is a non-[[supergiant]], fast-rotating Be type star with [[emission line]]s indicating [[luminosity class]] III-V. Most BeXRBs have eccentric [[orbit]]s and contain a neutron star, confirmed through X-ray pulsations.<ref name=reig/>

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]] ({{val|p=~|e=38|ul=erg/s}}) and lasting multiple orbital periods. Persistent BeXRBs display lower X-ray luminosity, more stable [[light curve]]s with minor variability, slower rotating neutron stars with [[spin period]]s exceeding 200 seconds, and typically have wide, low-eccentricity orbits.<ref name=reig/>

== Physical properties ==

Be stars show emission [[spectral line]]s (particularly [[H-alpha|Hα]]) and [[infrared excess]] from their [[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.<ref name=reig/>

The neutron star truncates the Be star's disk through [[tidal force]]s. Evidence for this includes correlation between Hα [[equivalent width]] and orbital period, higher disk densities than in isolated Be stars, shorter V/R variability periods, and quantized [[infrared]] flux states matching resonant truncation radii. During [[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.<ref name=reig/>

BeXRBs exist in both the [[Milky Way]] and [[Magellanic Clouds]], with the [[Small Magellanic Cloud]] hosting many of these systems due to its lower [[metallicity]] and higher [[star formation]] rate.<ref name=reig/>

== 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 diagram]]s and narrow-slit [[spectroscopy]] are used to accurately identify these optical counterparts.<ref name=reig/>

== Notable BeXRBs ==
=== X Persei ===
{{main|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.<ref name=hui/>

=== LSI+61°303 ===
{{main|LS I +61 303}}
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.<ref name=Taylor/>  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]].<ref name=massi/>

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 (energy)|kT]]''&nbsp;≈ 10–20 keV) vs. (''kT''&nbsp;≤ 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]], [[Cygnus X-3|Cyg X-3]] and [[Circinus X-1|Cir X-1]].<ref name=Taylor/>

=== RX J0209.6-7427 ===
RX J0209.6-7427 is a BeXRB located in the [[Magellanic Bridge]].<ref>
{{cite journal
| author1 =Kahabka, P. |author2=Hilker, M.
| title = Discovery of an X-ray binary in the outer SMC wing
| journal = Astronomy and Astrophysics
| date = 2005
| volume = 435
| issue = 1
| pages = 9–16
| doi = 10.1051/0004-6361:20042408
| bibcode = 2005A&A...435....9K| doi-access = free
}}</ref> 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]].<ref>
{{cite journal
| author1 = Chandra, A. D.|author2=Roy, J. |author3=Agrawal, P. C.|author4=Choudhury, M.
| title = 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?
| journal = Monthly Notices of the Royal Astronomical Society
| date = 2020
| volume = 495
| issue = 3
| pages = 2664–2672
| doi = 10.1093/mnras/staa1041
|doi-access=free | bibcode =  2020MNRAS.495.2664C
|arxiv = 2004.04930}}</ref><ref>{{cite news |title=Ultra-bright X-ray source awakens near a galaxy not so far away |url=https://ras.ac.uk/news-and-press/research-highlights/ultra-bright-x-ray-source-awakens-near-galaxy-not-so-far-away |date=June 2020 |work=Royal Astronomical Society}}</ref><ref>{{cite news |title=Ultra-Bright Pulsar Awakens Next Door To The Milky Way After 26-Year Slumber |url=https://www.iflscience.com/space/ultrabright-pulsar-awakens-next-door-to-the-milky-way-after-26year-slumber/ |date= June 2020 |work=Alfredo Carpineti}}</ref>

=== 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.<ref>{{cite news |last1=Nowakowski |first1=Tomasz |title=Astronomers discover a rare eclipsing X-ray binary |url=https://phys.org/news/2024-03-astronomers-rare-eclipsing-ray-binary.html |access-date=28 March 2024 |work=phys.org |language=en}}</ref><ref>{{cite journal |last1=Gaudin |first1=Thomas M. |last2=Kennea |first2=Jamie A. |last3=Coe |first3=Malcolm J. |last4=Monageng |first4=Itumeleng M. |last5=Udalski |first5=Andrzej |last6=Townsend |first6=Lee J. |last7=Buckley |first7=David A. H. |last8=Evans |first8=Phil A. |title=Discovery of a Rare Eclipsing Be/X-ray Binary System, Swift J010902.6-723710 = SXP 182 |journal=The Astrophysical Journal |date=2024 |volume=965 |issue=1 |pages=L10 |doi=10.3847/2041-8213/ad354a |doi-access=free |arxiv=2403.05648 |bibcode=2024ApJ...965L..10G }}</ref>

==References==
{{reflist|refs=

<ref name=hui>{{cite journal|bibcode=2014AJ....148..113L|arxiv=1408.3542|title=Long-term Optical Observations of the Be/X-Ray Binary X Per|journal=The Astronomical Journal|volume=148|issue=6|pages=113|last1=Li|first1=Hui|last2=Yan|first2=Jingzhi|last3=Zhou|first3=Jianeng|last4=Liu|first4=Qingzhong|year=2014|doi=10.1088/0004-6256/148/6/113}}</ref>

<ref name=massi>{{cite journal|bibcode=2017MNRAS.468.3689M|arxiv=1704.01335|title=The black hole candidate LS I +61°0303|journal=Monthly Notices of the Royal Astronomical Society |volume=468|issue=3|pages=3689|last1=Massi|first1=M|last2=Migliari|first2=S|last3=Chernyakova|first3=M|year=2017|doi=10.1093/mnras/stx778|doi-access=free }}</ref>

<ref name=reig>{{cite journal|bibcode=2011Ap&SS.332....1R|title=Be/X-ray binaries|journal=Astrophysics and Space Science|volume=332|pages=1–29|last1=Reig|first1=Pablo|year=2011|issue=1 |doi=10.1007/s10509-010-0575-8|arxiv=1101.5036}}</ref>

<ref name=Taylor>{{ cite journal |vauthors=Taylor AR, Young G, Peracaula M, Kenny HT, Gregory PC |title=An X-ray outburst from the radio emitting X-ray binary LSI+61°303 |journal=Astron. Astrophys. |year=1996 |volume=305 |pages=817–24 |bibcode=1996A&A...305..817T }}</ref>

}}

== Further reading ==
* {{cite journal |last1=Liu |first1=Boyuan |last2=Sartorio |first2=Nina S |last3=Izzard |first3=Robert G |last4=Fialkov |first4=Anastasia |title=Population synthesis of Be X-ray binaries: metallicity dependence of total X-ray outputs |journal=Monthly Notices of the Royal Astronomical Society |date=27 November 2023 |volume=527 |issue=3 |pages=5023–5048 |doi=10.1093/mnras/stad3475|doi-access=free |arxiv=2308.06154 }}
* {{cite conference |last1=Coe |first1=M. J. |title=Be stars in X-ray binary systems |journal=The be Phenomenon in Early-Type Stars, IAU Colloquium 175, ASP Conference Proceedings |date=2000 |volume=214 |page=656 |url=https://adsabs.harvard.edu/full/2000ASPC..214..656C |publisher=Astronomical Society of the Pacific |arxiv=astro-ph/9911272 |bibcode=2000ASPC..214..656C |isbn=1-58381-045-5}}

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

{{Neutron star}}

{{DEFAULTSORT:Be X-Ray Binaries}}

[[Category:X-ray binaries|*Be X-ray binary]]
[[Category:Be stars|*Be X-ray binary]]