Evolution of intermediate-mass X-ray binaries driven by magnetic braking of Ap/Bp stars: I. Ultracompact X-ray binaries

Journal article

It is generally believed that ultracompact X-ray binaries (UCXBs) evolved from binaries consisting of a neutron star accreting from a low-mass white dwarf(WD) or helium star where mass transfer is driven by gravitational radiation. However, the standard WD evolutionary channel cannot produce the relatively long-period (40–60 minutes) UCXBs with ahigh time-averaged mass-transfer rate. In this work, we explore an alternative evolutionary route toward UCXBs, where the companions evolve from intermediate-mass Ap/Bp stars with an anomalously strong magnetic field (100–10,000 G). Including the magnetic braking caused by the coupling between the magneticfield and an irradiation-driven wind induced by the X-rayflux from the accreting component, we show that intermediate-mass X-ray binaries(IMXBs)can evolve into UCXBs. Using theMESAcode, we have calculated evolutionary sequences for a large number of IMXBs. The simulated results indicate that, for a small wind-driving efficiency f=10^−5, the anomalous magnetic braking can drive IMXBs to an ultra-short period of 11 minutes. Comparing our simulated results with the observed parameters of 15identified UCXBs, the anomalous magnetic braking evolutionary channel can account for the formation of seven and eight sources with f = 10^-3, and 10^−5, respectively. In particular, a relatively large value of f canfit three of the long-period, persistent sources with ahigh mass-transfer rate. Though the proportion of Ap/Bp stars in intermediate-mass stars is only 5%, the lifetime of the UCXB phase is 2 Gyr, producing a relatively high number of observable systems, making this an alternative evolutionary channel for the formation of UCXBs.

Authors: Chen, W; Podsiadlowski, P

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Institute of Physics
• Journal: Astrophysical Journal
• Volume: 830
• Issue: 2
• Pages: 131
• Publication date: 2016-10-01
• Acceptance date: 2016-08-08
• DOI: 10.3847/0004-637X/830/2/131
• ISSN: 1538-4357
• Keywords: stars: mass-loss; X-rays: binaries; stars: evolution; binaries: general