Proposed wavelength measurements of silicon x-ray spectra: Application to vela X-1

When a stellar wind from a massive star is captured by a neutron star in close orbit, gravitational potential energy is converted into hard X-radiation near the surface of the neutron star. The X-radiation, in turn, modifies the wind through heating and photoionization, which affects the dynamics of mass capture. We have begun a project to further elucidate this process, which involves time-dependent three-dimensional hydrodynamics, large-scale atomic physics calculations, and radiation transport, integrated in an attempt to derive a self-consistent "first principles" description of the mass transfer process. We anticipate that the high-resolution silicon X-ray spectrum, produced by innershell photoionization and photoexcitation, as measured by the Chandra observatory, will provide benchmarks for these calculations. However, theoretical wavelengths, which are required in order to draw inferences concerning the velocity field of the wind, are uncertain at the level of the likely Doppler shifts in the stellar wind. EBIT measurements could lead to a reliable set of wavelengths, thereby providing observational constraints on the physics that powers some of the brightest X-ray sources in the Galaxy.