Photometric study of the supersoft X-ray source V Sge based on TESS data

Abstract

ABSTRACT V Sge is a unique eclipsing cataclysmic binary and a recognized supersoft X-ray source (SSS). By collecting available data on eclipse timings of V Sge, including our observations, we compiled and obtained an O − C diagram to analyse the orbital period variations of V Sge. The O − C curve displays a downward parabolic course, which implies that the orbital period is decreasing continuously at a rate of dP/dt = −1.92(2) × 10−7 d yr−1 = −0.0166(2) s yr−1. The continuously decreasing orbital period may be mainly caused by mass transfer from the more massive secondary component during its evolution on the Kelvin–Helmholtz time-scale, as well as mass loss from the accretion disc surrounding the white dwarf. Therefore, we estimate that the mass-accretion rate is less than $2.667\times 10^{-7}\, {\rm M}_{\odot }\, \mathrm{yr}^{-1}$. It can be roughly estimated that V Sge might reach the Chandrasekhar limit in at least 1 Myr, then might explode as a type Ia supernova. In addition, the more continuous TESS light curves show peculiar variations, that is, there is a great difference in brightness between two adjacent orbital periods. We speculate that this may be mainly caused by the extinction effect of the uneven circumstellar matter.