Recently, LIU Dongdong and WANG Bo from Yunnan Observatories of the Chinese Academy of Sciences (CAS) explain the formation of a special type of low-mass X-ray binaries through the accretion-induced collapse (AIC)  of white-dwarfs (WDs). This work was recently published in Monthly Notices of the Royal Astronomical Society.

Scientists have discovered a class of low-mass X-ray binary, which consists of a strong magnetic field, a slow spin neutron star (NS) and an extremely low mass companion (less than 0.1 solar mass). However, there exists a contradiction to explain its formation: on the one hand, the ultra-light companions and close orbits indicate that a considerable dynamical stable mass-transfer process has taken place between these binaries (as a dynamical unstable mass-transfer is more likely to lead to a merger during the common envelope process for such small mass-ratio binaries). On the other hand, the slow spin and relatively strong magnetic fields of these NSs both imply that the NSs are newly burned without accreting large amount of material.

It has been suggested that the formation of these newly burn NSs with ultra-light companions might be explained by the AIC events with surviving donors, in which the considerable dynamical stable mass-transfer process occurs in the WD binary stage, and the newly formed NSs accompanying with ultra-light stars are detected just after the AIC process and before or at the beginning of the recycling process.

However, there has always been a problem with this theoretical explanation: previous studies on the progenitors of the single-degenerate model of the AIC supernovae predicted extremely more massive companions after the AIC process, which cannot explain the formation of the low-mass X-ray binaries mentioned above.

To solve this problem, the researchers studied the oxygen neon WD + helium WD channel for the formation of AIC supernovae, successfully explaining the formation problem for the newly formed NS + extremely low mass companion systems. When the companion star fills the Roche lobe again, it will transfer material to the NS, and then the binary system behaves as an ultra-compact X-ray binary (UCXB).

UCXBs are important gravitational wave sources. 4U 1626-67 is a binary consisting of a strong magnetic field, slow spin NS and an extremely low mass companion, which is also a UCXB. Although there are some other inconsistent properties, the detected companion mass and orbital period of 4U 1626-67 can be reproduced by this channel. In addition, combined with previous asteroseismology results, the researchers speculate that the UCXB source, XTE J1751？305 also originate from the evolutionary channel with helium WD donors.

Contact:
LIU Dongdong
Yunnan Observatories, CAS
Email: liudongdong@ynao.ac.cn