Type I outbursts in low eccentricity Be/X-ray binaries

Abstract

Type I outbursts in Be/X-ray binaries are usually associated with the eccentricity of the binary orbit. The neutron star accretes gas from the outer parts of the decretion disk around the Be star at each periastron passage. However, this mechanism cannot explain type I outbursts that have been observed in nearly circular orbit Be/X-ray binaries. With hydrodynamical simulations and analytic estimates we find that in a circular orbit binary, a nearly coplanar disk around the Be star can become eccentric. The extreme mass ratio of the binary leads to the presence of the 3:1 Lindblad resonance inside the Be star disk and this drives eccentricity growth. Therefore the neutron star can capture material each time it approaches the disk apastron, on a timescale up to a few percent longer than the orbital period. We have found a new application of this mechanism that is able to explain the observed type I outbursts in low eccentricity Be/X-ray binaries.

6 pages, 4 figures, Accepted for publication in ApJL

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• Retrograde discs around one component of a binary are unstable to tilting
• Disentangling the neighbouring pulsars SXP 15.3 and SXP 305
• Disc breaking through forced eccentricity growth
• Astrophysical parameters of LS 437 and the nature of X0726-260
• Non-periodic type I Be/X-ray binary outbursts