The First OGLE-discovered Ultracompact X-Ray Binary is an Intermediate Polar

Abstract

Abstract The variable source OGLE-UCXB-01 is the first OGLE-discovered ultracompact X-ray binary (UCXB). The 12 yr long-term OGLE optical photometry of this source shows a period of P = 12.8 min and a fast period-decreasing rate P ̇ = − 9.2 × 10 − 11 s s−1. At a luminosity of L X ≈ 4 × 10 33 erg s−1, its X-ray emission is also variable and correlated with the optical variability. To determine the nature of this variable source, specifically the masses and types of its binary components, we first consider an attractive possibility that the optical variation is due to the secondary’s ellipsoidal variation and a strong gravitational wave emission drives the orbital decay. However, we cannot find an allowable solution to the secondary that simultaneously satisfies the three constraints: an ultratight orbit, the bright absolute magnitude, and the large-amplitude brightness variation. Moreover, the inferred mass-transfer rate is too high. This scenario is therefore ruled out. We then find the system is fully consistent with an “intermediate polar” model, in which the optical and X-ray emission comes from a magnetized white dwarf (WD) accreting from a low-mass (≲ 0.7 M ⊙ ) main-sequence secondary. The observed period decay is the accretion-driven spin-up of the WD. The WD spin period is 12.8 minutes and the orbital period is shorter than 10 hr. The method presented here can be applied to other UCXB candidates or impostors with only time-domain data available.