X-Ray Line Profiles from Parameterized Emission Within an Accelerating Stellar Wind

Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line profiles for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply defined co-moving-frame resonance wavelength, which is Doppler-shifted by a stellar wind outflow parameterized by a ‘beta’ velocity law, v(r) = v∞(1−R∗/r). Above some initial onset radius Ro for X-ray emission, the radial variation of the emission filling factor is assumed to decline as a power-law in radius, f(r) ∼ r. The computed emission profiles also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth τ∗. In terms of a wavelength shift from line-center scaled in units of the wind terminal speed v∞, we present normalized X-ray line profiles for various combinations of the parameters β, τ∗, q and Ro, and including also the effect of instrumental broadening as characterized by a Gaussian with a parameterized width σ. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between “coronal” and “wind-shock” scenarios. In particular, we note that in profiles observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.