Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?

Abstract

Gamma~Cas stars are early-type Be stars that exhibit an unusually hard and bright thermal X-ray emission. One of the proposed scenarios to explain these properties postulates the existence of a neutron star companion in the propeller stage, during which the magnetosphere of a rapidly rotating neutron star repels infalling material. To test this model, we examined the fluorescent Fe K$α$ emission line at $\sim 6.4$\,keV in the X-ray spectra of $γ$~Cas stars, which offers a powerful diagnostic of both the primary source of hard X-rays and the reprocessing material. We computed synthetic line profiles of the fluorescent Fe K$α$ emission line in the framework of the propelling neutron star scenario. Two reservoirs of material contribute to the fluorescence in this case: the Be circumstellar decretion disk and a shell of cool material that surrounds the shell of X-ray-emitting plasma around the putative propelling neutron star. We analysed the synthetic line profiles and expected equivalent widths of the lines for three well-studied $γ$~Cas stars. The predicted line strengths fall short of the observed values by at least an order of magnitude. Pushing the model parameters to reproduce the observed line strengths led to column densities towards the primary X-ray source that exceed the observationally determined values by typically a factor of 20, and would further imply a higher X-ray luminosity than observed. The strengths of the observed Fe K$α$ fluorescent emission lines in $γ$~Cas stars are inconsistent with the expected properties of a propeller scenario as proposed in the literature.