Effects of Irradiation on the Evolution of Ultra-compact X-ray Binaries

TL;DR

This study uses stellar evolution simulations to explore how irradiation influences ultra-compact X-ray binary evolution, revealing effects mainly during transient outbursts and explaining observed persistent sources.

Contribution

It provides new insights into the role of irradiation in UCXB evolution, especially during outburst phases, with implications for observed persistent sources.

Findings

Irradiation has minimal impact on persistent UCXBs due to large WD masses.

During outbursts, irradiation causes WD expansion and increases mass-transfer rates.

Transient UCXBs with smaller WD masses experience prolonged irradiation effects.

Abstract

Using the Modules for Experiments in Stellar Astrophysics code, we investigate the influences of irradiation on ultra-compact X-ray binary (UCXB) evolution. Although the persistent UCXBs have short orbital periods which result in high irradiation flux, the irradiation hardly affects the evolution of persistent sources because the WDs in these binaries have large masses which 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 WD. Then, the irradiation triggers high mass-transfer rates, which can last several days for the transient sources with WDs whose masses are larger than $\sim0.015 M_\odot$ or several hundred years for these sources with WDs whose masses are less than $\sim0.012 M_\odot$. The observed three persistent UCXBs, XTE J0929-314, 4U 1916-05 and SWIFT J1756.9-2508, may belong to the latter.