Recently, PhD candidate ZHAO Weitao and Prof. MENG Xiangcun, at Yunnan Observatories, Chinese Academy of Sciences (CAS), have made new progress in the field of supersoft X-ray sources. By simulating the feedback of the white dwarf (WD) to the periodic mass transfer driven by the X-ray irradiation on companion, they reproduced the quasi-periodic light curve of the supersoft X-ray source, which provides a new way to explain the origin of the quasi-periodic light curve of the supersoft X-ray source. The research results were published in the international scientific journal Astrophysics and Astrophysics.

Type Ia supernovae are now being used as standard candles to measure cosmological parameters and thus examine the evolution of the dark energy equation of state with time. However, the progenitors of type Ia supernovae are still unknown, which may hinder the development of accurate cosmology. Supersoft X-ray sources are considered the most likely progenitor objects for type Ia supernovae.

Supersoft X-ray source consists of a WD and a massive main-sequence companion star. The WD accretes materials from its companion star and increases its mass until it reaches the Chandrasekhar mass limit, causing a type Ia supernova explosion. The light curves of supersoft X-ray sources show a quasi-periodic variability of bright and dim. However, the reason of such quasi-periodic variation of the light curve of supersoft X-ray sources is still unclear.

Previous studies on the reason of this quasi-periodic light curve ignored the effect of supersoft X-rays on the companion star. The X-ray irradiation is expected to heat the companion star and change its effective surface boundary condition, which then attenuates the mass transfer rate of the binary system. Therefore, the researchers proposed that periodically supersoft X-rays from a white dwarf irradiate its companion star, which leads to the expansion and contraction of the companion star, and then a periodical mass transfer rate. As a result, the binary mass transfer rate increases and decreases periodically.

For this study, the researchers used Modules for Experiments in Stellar Astrophysics (MESA) code to carry out 1D simulations, and adopted a periodic jagged accretion rate onto the WD instead of the periodic variability in the mass transfer rate.

Due to periodic variability of the mass accretion rate onto the WD, the WD photosphere periodically expands and constants, which nicely reproduces the light curve of the supersoft X-ray source. This result provides a new way to explain the origin of quasi-periodic light curves in supersoft X-ray sources, and also provides a new research idea for the study of type Ia supernova progenitors.

The research was supported by the CAS West Light Foundation and the National Science Foundation of China.

Contact:
ZHAO Weitao
Yunnan Observatories, CAS
Email: zhaoweitao@ynao.ac.cn