The nature of very faint X-ray binaries; hints from light curves

TL;DR

This paper investigates the properties and possible origins of very faint X-ray binaries (VFXBs) by analyzing their light curves, proposing explanations for their different decay behaviors, and suggesting they may include transitional millisecond pulsars.

Contribution

The study provides a detailed analysis of VFXB light curves, linking decay types to specific system characteristics and proposing a new connection to transitional millisecond pulsars.

Findings

Linear decay VFXBs may be partial drainings of normal transients' discs.

Exponential decay VFXBs likely involve old, short-period systems with low-mass white dwarfs or brown dwarfs.

Persistent VFXBs could be magnetospheric choked systems, possibly transitional millisecond pulsars.

Abstract

Very faint X-ray binaries (VFXBs), defined as having peak luminosities Lx of 10^34-10^36 erg/s, have been uncovered in significant numbers, but remain poorly understood. We analyse three published outburst light curves of two transient VFXBs using the exponential and linear decay formalism of King and Ritter (1998). The decay timescales and brink luminosities suggest orbital periods of order 1 hour. We review various estimates of VFXB properties, and compare these with suggested explanations of the nature of VFXBs. We suggest that: 1) VFXB outbursts showing linear decays might be explained as partial drainings of the disc of "normal" X-ray transients, and many VFXB outbursts may belong to this category; 2) VFXB outbursts showing exponential decays are best explained by old, short-period systems involving mass transfer from a low-mass white dwarf or brown dwarf; 3) persistent (or quasi-persistent) VFXBs, which maintain an Lx of 10^34-10^35 erg/s for years, may be explained by magnetospheric choking of the accretion flow in a propeller effect, permitting a small portion of the flow to accrete onto the neutron star's surface. We thus predict that (quasi-)persistent VFXBs may also be transitional millisecond pulsars, turning on as millisecond radio pulsars when their Lx drops below 10^32 erg/s.