Long-term photometric behavior and orbital period variations in the supersoft X-ray source V617 Sgr

Abstract

V617 Sgr is one of the Galactic counterparts of supersoft X-ray sources as a member of the V Sagittae class, where a massive white dwarf is accreting material from its stellar companion and forming an accretion disk. Since 2014, we have monitored it for about nine years with the 2.15 m telescope at CASLEO. It is found that the light curve shows obviously fluctuations and flickering, which imply the presence of active hot-spots on the accretion disk. Meanwhile, the flat-bottomed depressions can be explained by the presence of circumbinary material. By collecting all available eclipse times from the literature together with 149 newly determined times of light minimum, we constructed the O-C diagram and analyzed the variations in the orbital period of the eclipsing binary V617 Sgr. It is confirmed that the orbital period is increasing continuously with a rate of P ̇=+ 1. 68 (3)×10−7 d⋅ yr−1 ̇. A cyclic variation is detected to be superimposed on the period increase with an amplitude of 4.6 min and a period of 26 yr. The continuous period increase is largely due to mass transfer from the donor secondary on its Kelvin–Helmholtz time-scale, while wind mass loss from the accretion disk surrounding the white dwarf may also contribute on the period change. In this case, the mass-transfer rate of V617 Sgr is estimated to be in the range of about 0.575×10−6 to 2.25×10−6 M⊙ yr−1 and the mass-accretion rate is less than 1.59×10−6 M⊙ yr−1. The cyclic change can be explained by the presence of a third body or magnetic activity cycles of the secondary.