Be–white dwarf X-ray binary systems (BeWDs) are a rare type ofX-ray binary consisting of awhite dwarf that accretes matter from a rapidly-rotatingBe star. These systems form through binary evolution where mass transfer spins up the accretor to become a Be star while the donor evolves into a white dwarf.[2]
BeWDs probably originate from a Be star and asubdwarf O orB star binaries.[3] Population synthesis models indicate these systems can evolve through two primary pathways:
- Approximately 60-70% merge into red giants that observationally look likeluminous red novae.[3]
- About 30-40% evolve into double white dwarf systems that may be detectable as gravitational wave sources byLaser Interferometer Space Antenna (LISA), and will be its "most likely gravitational wave source".[3]
The formation requires specific initial conditions: the primary must transfer sufficient mass to spin up the secondary to Be star velocities without triggering common envelope evolution.Tidal synchronization mechanisms explain the observed lack of BeWDs with orbital periods shorter than 17 days.[4]
BeWDs can be identified by several features:[2]
- Supersoft X-ray emission (kT ~ 0.1 keV)
- X-rayluminosities of 1033-1038 erg/s
- Deepnitrogen (0.67 keV) andoxygen (0.87 keV) absorption edges in X-ray spectra
The white dwarfs in these systems tend to be massive (0.9-1.35M☉) with surface temperatures of 20,000-40,000 K.[4] Detection is challenging as the white dwarf is often embedded within the Be star's decretion disk, absorbing most extreme-UV and soft X-ray photons.[4]
Some studies suggest thatγ Cas stars, a subgroup of Be stars exhibiting bright X-ray emission, likely have white dwarf companions rather than hot subdwarf stars or main sequence stars, as interferometric observations show no detectable companion flux while the systems' properties match theoretical predictions for Be+WD binaries.[5]
Despite theoretical predictions that BeWDs should be 7 times more common thanBe-neutron star systems,[2][4] only 8 have been confirmed as of 2025. According to different numerical models, 40 to 80% of Be stars should have white dwarf companions.[6]
| BeWD | Porb (day) | LX (erg s−1) | MWD (M☉) | Be star | Galaxy | Notes and References |
|---|---|---|---|---|---|---|
| XMMU J052016.0-692505 | 510 or 1020 | 1034-1038 | 0.9-1.0 | B0-B3e | LMC | observed byXMM-Newton[7] |
| XMMU J010147-715550 | 1264 | ~4.4 × 1033 | 1.0 | O7IIIe-B0Ie | SMC | observed byXMM-Newton[8][9] |
| MAXI J0158-744 | ... | >1037; 1040 (peak) | 1.35 | B1-2IIIe | SMC | observed byMAXI[10][11] |
| SWIFT J011511.0-725611 | 17.402 | 2 × 1033-3.3 × 1036 | 1.2 | O9IIIe | SMC | observed bySwift[12] |
| SWIFT J004427.3-734801 | 21.5 | 5.7-2.9 × 1036 | ... | O9Ve-B2IIIe | SMC | observed bySwift[13] |
| RX J0527.8-6954 | ... | 4-9 × 1036 | ... | B5eV | LMC | observed by theGemini South telescope[14] |
| CXOU J005245.0−722844 | 17.55 (shortening to 17.14) | 6.51 × 1038 | 1.2 | O9V-B0Ve | SMC | discovered "via a very fast, super-Eddington X-ray outburst event"[15] |
| EP J005245.1−722843 | 17.55 | ~4 × 1038 | ~1.2 (likely Ne-O WD) | O9V-B0Ve | SMC | observed by theEinstein Probe[2] |
All identified systems are located in theMagellanic Clouds rather than theMilky Way, possibly due to lowerextinction rates allowing easier detection of soft X-rays, or because of the differentmetallicity of the Magellanic Clouds which may be related to the formation of BeWDs.[12]
References
edit- ^"Einstein Probe catches X-ray odd couple".www.esa.int.
- ^abcdMarino, A.; et al. (2025)."Einstein Probe Discovery of EP J005245.1−722843: A Rare Be–White Dwarf Binary in the Small Magellanic Cloud?".The Astrophysical Journal Letters.980 (2): L36.doi:10.3847/2041-8213/ad9580.
- ^abcdZhu, Chun-Hua; Lü, Guo-Liang; Lu, Xi-Zhen; He, Jie (2023)."Formation and Destiny of White Dwarf and be Star Binaries".Research in Astronomy and Astrophysics.23 (2): 025021.arXiv:2304.02615.Bibcode:2023RAA....23b5021Z.doi:10.1088/1674-4527/acafc7.
- ^abcdRaguzova, N. V. (2001)."Population synthesis of Be/White dwarf binaries in the Galaxy".Astronomy & Astrophysics.367 (3):848–858.Bibcode:2001A&A...367..848R.doi:10.1051/0004-6361:20000348.
- ^Gies, Douglas R.; Wang, Luqian; Klement, Robert (2023)."Gamma Cas Stars as Be+White Dwarf Binary Systems".The Astrophysical Journal Letters.942 (1): L6.arXiv:2212.06916.Bibcode:2023ApJ...942L...6G.doi:10.3847/2041-8213/acaaa1.
- ^
- 46% perRaguzova, Natalya V. (2000). "The Evolutionary Evidence for Most be Stars Being Paired with Hot White Dwarfs".International Astronomical Union Colloquium.175:693–696.doi:10.1017/S0252921100056761.
- 70% perRaguzova, Natalya (2002)."Population Synthesis of Binary be Stars with Degenerate Companions in the Galaxy".Astrophysics and Space Science.281 (3):641–649.Bibcode:2002Ap&SS.281..641R.doi:10.1023/A:1015819708250.
- 70-80% perRaguzova, N. V. (2003)."Population synthesis of rapidly rotating main-sequence stars with companions".Astronomy Reports.47 (5):401–412.Bibcode:2003ARep...47..401R.doi:10.1134/1.1575854.
- ^Kahabka, P.; Haberl, F.; Payne, J. L.; Filipović, M. D. (2006)."The super-soft source XMMU J052016.0-692505 in the LMC".Astronomy & Astrophysics.458:285–292.doi:10.1051/0004-6361:20065490.
- ^Sturm, R.; Haberl, F.; Pietsch, W.; Coe, M. J.; Mereghetti, S.; La Palombara, N.; Owen, R. A.; Udalski, A. (2012)."A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/White dwarf system in the SMC?".Astronomy & Astrophysics.537: A76.arXiv:1112.0176.Bibcode:2012A&A...537A..76S.doi:10.1051/0004-6361/201117789.
- ^Sturm, Richard K. N. (2012).An X-ray investigation of the Small Magellanic Cloud with XMM-Newton (Thesis).Bibcode:2012PhDT.......631S.
- ^Li, K. L.; Kong, Albert K. H.; Charles, P. A.; Lu, Ting-Ni; Bartlett, E. S.; Coe, M. J.; McBride, V.; Rajoelimanana, A.; Udalski, A.; Masetti, N.; Franzen, Thomas (2012). "A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI J0158-744".The Astrophysical Journal.761 (2): 99.arXiv:1207.5023.Bibcode:2012ApJ...761...99L.doi:10.1088/0004-637X/761/2/99.
- ^Morii, M.; et al. (2013). "Extraordinary Luminous Soft X-Ray Transient MAXI J0158-744 as an Ignition of a Nova on a Very Massive O-Ne White Dwarf".The Astrophysical Journal.779 (2): 118.arXiv:1310.1175.Bibcode:2013ApJ...779..118M.doi:10.1088/0004-637X/779/2/118.
- ^abKennea, J. A.; Coe, M. J.; Evans, P. A.; Townsend, L. J.; Campbell, Z. A.; Udalski, A. (2021)."Swift J011511.0-725611: Discovery of a rare be star/White dwarf binary system in the SMC".Monthly Notices of the Royal Astronomical Society.508:781–788.arXiv:2109.05307.doi:10.1093/mnras/stab2632.
- ^Coe, M. J.; Kennea, J. A.; Evans, P. A.; Udalski, A. (2020)."Swift J004427.3−734801 – a probable Be/White dwarf system in the Small Magellanic Cloud".Monthly Notices of the Royal Astronomical Society: Letters.497:L50 –L55.arXiv:2005.02891.doi:10.1093/mnrasl/slaa112.
- ^Oliveira, A. S.; Steiner, J. E.; Ricci, T. V.; Menezes, R. B.; Borges, B. W. (2010)."Optical identification of the transient supersoft X-ray source RX J0527.8-6954, in the LMC".Astronomy and Astrophysics.517: L5.arXiv:1006.4820.Bibcode:2010A&A...517L...5O.doi:10.1051/0004-6361/201014773.
- ^M, Gaudin; J, Coe; A, Kennea; M, Monageng; Buckley, D A H.; A, Udalski; A, Evans (4 October 2024)."CXOU J005245.0−722844: Discovery of a be star/White dwarf binary system in the SMC via a very fast, super-Eddington X-ray outburst event".Monthly Notices of the Royal Astronomical Society.534 (3):1937–1948.arXiv:2408.01388.doi:10.1093/mnras/stae2176.