@article{OskinovaFeldmeierKretschmar2012,
  author    = {Oskinova, Lida and Feldmeier, Achim and Kretschmar, Peter},
  title     = {Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {421},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {4},
  publisher = {Wiley-Blackwell},
  address   = {Malden},
  issn      = {0035-8711},
  doi       = {10.1111/j.1365-2966.2012.20507.x},
  pages     = {2820 -- 2831},
  year      = {2012},
  abstract  = {The clumping of massive star winds is an established paradigm, which is confirmed by multiple lines of evidence and is supported by stellar wind theory. The purpose of this paper is to bridge the gap between detailed models of inhomogeneous stellar winds in single stars and the phenomenological description of donor winds in supergiant high-mass X-ray binaries (HMXBs). We use the results from time-dependent hydrodynamical models of the instability in the line-driven wind of a massive supergiant star to derive the time-dependent accretion rate on to a compact object in the BondiHoyleLyttleton approximation. The strong density and velocity fluctuations in the wind result in strong variability of the synthetic X-ray light curves. The model predicts a large-scale X-ray variability, up to eight orders of magnitude, on relatively short time-scales. The apparent lack of evidence for such strong variability in the observed HMXBs indicates that the details of the accretion process act to reduce the variability resulting from the stellar wind velocity and density jumps.},
  language  = {en}
}
@article{BozzoOskinovaFeldmeieretal.2016,
  author    = {Bozzo, Enrico and Oskinova, Lida and Feldmeier, Achim and Falanga, M.},
  title     = {Clumpy wind accretion in supergiant neutron star high mass X-ray binaries},
  series = {BMC neuroscience},
  volume    = {589},
  journal   = {BMC neuroscience},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201628341},
  pages     = {369 -- 389},
  year      = {2016},
  abstract  = {The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.},
  language  = {en}
}