An observational test to determine the presence of intrinsic X‐ray absorption

A key factor in the analysis of X‐ray data is determining the true nature of the X‐ray absorption, i.e., the relative strengths of the intrinsic or ‘‘local’’ and ISM absorption components. In O stars, this local component is in addition to the fixed ISM absorption and refers to the X‐ray attenuation caused by the massive, highly ionized stellar wind. By comparing IPC and PSPC hardness ratios, we show how this correlation can be used to verify the presence of the local absorption component. We conclude that the observed X‐ray characteristics inherent in O stars are strongly influenced by the local absorption component. Although only demonstrated for O stars, this test should also be applicable to any X‐ray source possessing IPC and PSPC data.