Recently, a research team led by Prof. WANG Bo from Yunnan Observatories of the Chinese Academy of Sciences proposed a new model to explain ultracompact X-ray binaries (UCXBs) with long orbital periods, and they found that UCXBs can be identified by their locations in the mass-transfer rate versus the orbital period diagram. This work was recently published in Monthly Notices of the Royal Astronomical Society.
UCXBs are a sub-type of low-mass X-ray binaries, containing a compact accretor and a hydrogen-deficient mass-donor with an ultra-short orbital period (usually less than 1 hour). They are a kind of accretion-powered X-ray sources, in which the accretor could be a neutron star (NS) that is accreting matter from a mass-donor through Roche-lobe overflow (RLOF).
UCXBs play an important role in broad aspects of astrophysics. UCXBs have been thought to be strong continuous gravitational wave (GW) sources in the low-frequency region, which can be detected by the space GW detectors, such as Laser Interferometer Space Antenna (LISA), Taiji, and TianQin, etc. Besides, UCXBs provide important constraints on the binary evolution, such as the angular-momentum loss mechanisms, the common-envelope evolution and the mass-accretion process of compact objects, etc. Moreover, UCXBs are excellent astrophysical laboratories, since they are interesting X-ray sources and also the combination of ultra-short orbital periods, compact accretors and mass-donors with different chemical compositions. UCXBs have been proposed to be progenitor candidates of millisecond radio pulsars.
Several channels for the formation of UCXBs have been proposed so far. In this work, the researchers carried out a systematic study on the He star donor channel, in which a NS accretes matter from a He star through RLOF, where the mass-transfer is driven by the GW radiation. Firstly, the researchers followed the long-term evolution of the NS+He star binaries by employing the stellar evolution code Modules for Experiments in Stellar Astrophysics, and thereby obtained the initial parameter spaces for the production of UCXBs. The researchers then used these results to perform a detailed binary population synthesis approach to obtain the Galactic rates of UCXBs through this channel.
The researchers estimate the Galactic rates of UCXBs appearing as LISA sources to be about 3 to 12 per Myr through this channel, and the number of such UCXB-LISA sources in the Galaxy can reach about 1-26 calibrated by observations. The present work indicates that the He star donor channel may contribute significantly to the Galactic UCXB formation rate. The researchers found that the evolutionary tracks of UCXBs through this channel can account for the location of the five transient sources with relatively long orbital periods quite well. The researchers also found that such UCXBs can be identified by their locations in the mass-transfer rate versus the orbital period diagram.
After the UCXB stage, the evolved NS systems studied in this work will show as millisecond radio pulsars, eventually forming single millisecond radio pulsars or pulsar+planet-like systems.
Contact:
WANG Bo, Yunnan Observatories, CAS
wangbo@ynao.ac.cn
