The first OGLE-discovered ultracompact X-ray binary is an intermediate polar

TL;DR

This paper identifies the first OGLE-discovered ultracompact X-ray binary as an intermediate polar, revealing a magnetized white dwarf accreting from a low-mass secondary with a 12.8-minute spin period, challenging previous models.

Contribution

It demonstrates that the ultracompact X-ray binary is best explained as an intermediate polar system, providing a new classification for this source and a method applicable to similar objects.

Findings

The system is consistent with an intermediate polar model.

The white dwarf's spin period is 12.8 minutes.

The orbital period is shorter than 10 hours.

Abstract

The variable source OGLE-UCXB-01 is the first OGLE-discovered ultracompact X-ray binary (UCXB). The 12-year long-term OGLE optical photometry of this source shows a period of P= 12.8 min and a fast period decreasing rate Pdot= -9.2E-11 s s^-1. At a luminosity of L_X ~ 4E33 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 consider first 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 can not find an allowable solution to the secondary that satisfies simultaneously the three constraints: an ultra-tight orbit, the bright absolute magnitude, and the large amplitude of the 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_sun) main-sequence secondary. The observed period decay is the accretion-driven spin-up of the WD. The WD spin period is 12.8 min and the orbital period is shorter than 10 hr. The method presented here can be applied to other UCXB candidates or impostors with time-domain data available only.