V1082-Sgr: A magnetic cataclysmic variable with a lobe-filling companion star

Abstract

V1082 Sgr is a cataclysmic variable with accretion luminosity above 10³⁴ erg s^-1, indicating a mass-transfer rate above 10^{-9} M_{⊙ } yr^-1. However, its K-type companion was suggested to be underfilling its Roche lobe (RL), making the high mass-transfer rate a mystery. In this work we propose a possible model to explain this discrepancy. The system is proposed to be an intermediate polar, with its K-type companion filling its RL. The mass of the white dwarf star is evaluated to be 0.77± 0.11 M_{⊙ } from both X-ray continuum fitting and Fe line flux ratio measurements. We make numerical simulations to search for the possible progenitors of the system. The results show that a binary with an initial 1.5-2.5 M_{⊙ } companion in a one to two day orbit (or an initial 1.0-1.4 M_{⊙ } companion in a 3.2-4.1 d orbit) may naturally evolve to a cataclysmic variable with a ∼ 0.55 ± 0.11 M_{⊙ }, Roche-lobe-filling companion in a 0.86 d orbit. The effective temperature of the donor star, the mass-transfer rate, and the derived V-band magnitude are all consistent with previous observations.