Mon. Not. R. Astron. Soc., 447, 3034-3043 (2015/March-2)
The nature of very faint X-ray binaries: hints from light curves.
HEINKE C.O., BAHRAMIAN A., DEGENAAR N. and WIJNANDS R.
Abstract (from CDS):
Very faint X-ray binaries (VFXBs), defined as having peak luminosities LX of 1034-1036 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 & 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 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.
© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society (2015)
accretion, accretion discs - X-rays: binaries - X-rays: individual: CXOGC J174540.0-290005 - X-rays: individual: XMM J174457-2850.3
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