Researchers led by Ph.D. candidate Zhao Weitao and Prof. Meng Xiangcun from the Chinese Academy of Sciences’ Yunnan Observatories reproduced the supersoft X-ray source’s quasi-periodic light curve by simulating the feedback of the white dwarf (WD) to the periodic mass transfer driven by X-ray irradiation on its companion.
The findings were published in Astronomy and Astrophysics.
Type Ia supernovae are now used as standard candles for measuring cosmological parameters and studying the evolution of the dark energy equation of state over time. The most likely progenitor objects for type Ia supernovae are supersoft X-ray sources.
The light curves of supersoft X-ray sources show a quasi-periodic variability of lightness and darkness. However, the reason of such quasi-periodic variation of the light curve of supersoft X-ray sources is still unclear.
A 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 lightness and darkness. However, the reason of such quasi-periodic variation of the light curve of supersoft X-ray sources is still unclear.
Previous studies on 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.
The researchers proposed in this study that the companion star be irradiated with supersoft X-rays on a regular basis, causing it to expand and contract. As a result, the binary mass transfer rate fluctuates on a regular basis.
They ran 1D simulations using the Modules for Experiments in Stellar Astrophysics (MESA) code, and instead of the periodic variability in the mass transfer rate, they used a periodic jagged accretion rate onto the WD.
The WD photosphere expands and contracts on a regular basis due to the periodic variability of the mass accretion rate onto the WD, which nicely reproduces the light curve of the supersoft X-ray source.
