Modeling broadband X-ray absorption of massive star winds

Maurice A. Leutenegger (1,2), David H. Cohen (3), Janos Zsarg'{o} (4,5), Erin M. Martell (6,3), James P. MacArthur (3), Stanley P. Owocki (7), Marc Gagn'{e} (8), D. John Hillier (5)

(1) Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD, USA
(2) NASA Postdoctoral Fellow
(3) Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA, USA
(4) Instituto Politecnico Nacional, Escuela Superior de Fisica y Matematicas, Mexico City, Mexico
(5) Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA, USA
(6) Department of Astronomy, University of Chicago, Chicago, IL, USA
(7) Bartol Research Institute, University of Delaware, Newark, DE, USA
(8) Department of Geology and Astronomy, West Chester University of Pennsylvania, West Chester, PA, USA

We present a method for computing the net transmission of X-rays emitted by shock-heated plasma distributed throughout a partially optically thick stellar wind from a massive star. We find the transmission by an exact integration of the formal solution, assuming the emitting plasma and absorbing plasma are mixed at a constant mass ratio above some minimum radius, below which there is assumed to be no emission. This model is more realistic than either the slab absorption associated with a corona at the base of the wind or the exospheric approximation that assumes all observed X-rays are emitted without attenuation from above the radius of optical depth unity. Our model is implemented in XSPEC as a pre-calculated table that can be coupled to a user-defined table of the wavelength dependent wind opacity. We provide a default wind opacity model that is more representative of real wind opacities than the commonly used neutral ISM tabulation. Preliminary modeling of textit{Chandra} grating data indicates that the X-ray hardness trend of OB stars with spectral subtype can largely be understood as a wind absorption effect.

Reference: Accepted by ApJ
Status: Manuscript has been accepted


Comments: Model available at