Journal Article

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

C. O. Heinke, A. Bahramian, N. Degenaar and R. Wijnands

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 447, issue 4, pages 3034-3043
ISSN: 0035-8711
Published online January 2015 | e-ISSN: 1365-2966 | DOI: https://dx.doi.org/10.1093/mnras/stu2652
The nature of very faint X-ray binaries: hints from light curves

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Very faint X-ray binaries (VFXBs), defined as having peak luminosities LX of 1034–1036 erg s−1, 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 & Ritter. The decay time-scales and brink luminosities suggest orbital periods of order 1 h. 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 1034–1035 erg s−1 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 on to 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 1032 erg s−1.

Keywords: accretion, accretion discs; X-rays: binaries; X-rays: individual: CXOGC J174540.0−290005; X-rays: individual: XMM J174457−2850.3

Journal Article.  7268 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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