Effects of Irradiation on the Evolution of Ultracompact X-Ray Binaries

Abstract

Using the Modules for Experiments in Stellar Astrophysics code, we investigate the influences of irradiation on ultracompact X-ray binary (UCXB) evolution. Although the persistent UCXBs have short orbital periods that result in high irradiation flux, the irradiation hardly affects the evolution of persistent sources because the white dwarfs (WDs) in these binaries have large masses that lead to very low irradiation depth. The irradiation has a significant effect on the transient sources during outburst phase. At the beginning of the outburst, high X-ray luminosity produces high radiation flux, which results in the significant expansion of WDs. Then, the irradiation triggers high mass-transfer rates, which can last several days for the transient sources with WDs whose masses are larger than or several hundred years for those sources with WDs whose masses are less than . The observed three persistent UCXBs, XTE J0929-314, 4U 1916-05, and SWIFT J1756.9-2508, may belong to the latter.