Photometric Analysis of the TESS Light Curve for the Asynchronous Polar V1500 Cyg

Abstract

After 40 yr of nova outburst, there are still some difficulties in completely understanding V1500 Cyg. The faintness of V1500 Cyg and its crowded field pose a challenge for high-precision photometry by TESS, while long-term intensive data from TESS provide the first opportunity to accurately model this binary system. The best-fitting model reveals that the high-temperature white dwarf illuminates the surface of its companion star, a low-mass secondary (M 2 ∼ 0.22 M ⊙), and the emission from the heated secondary star still dominates the luminosity of the system. The binary model has determined the white dwarf’s mass to be M 1 ∼ 1.4 M ⊙ and its orbital inclination i ∼ 60°. The residuals after the removal of the fitted model curve present variations in both the white dwarf’s spin period and the binary orbital period, confirming the previous claim that V1500 Cyg is still an asynchronous polar, and the observed spin period demonstrates that V1500 Cyg is approaching synchronicity with the orbital period following the prediction of the polarization ephemeris. The phasing of the spin humps indicates their cyclotron origin, and the residual orbital humps may be produced by the emission from the accretion stream. There is no evidence for the pole-switching effect, while the double-humped profile of the spin modulation suggests that the white dwarf may be accreting matter from a “ring” around it. Further simultaneous polarization and X-ray observations are urged to explore the accretion geometry of V1500 Cyg in detail.