The orbital period evolution of the supersoft X-ray source CAL 87

Abstract

CAL 87 is one of the best known supersoft X-ray sources. However, the measured masses, orbital period and orbital period evolution of CAL 87 cannot be addressed by the standard thermal-timescale mass-transfer model for supersoft X-ray sources. In this work we explore the orbital evolution of CAL 87 with both analytic and numerical methods. We demonstrate that the characteristics mentioned above can be naturally accounted for by the excited-wind-driven mass-transfer model.