A Chandra spectroscopic survey of persistent black hole candidates

We present results from observations of persistent black hole candidates with the High-Energy Transmission Gratings on board the Chandra X-Ray Observatory. The sources include LMC X-1, LMC X-3, GRS 1758 258, and Cyg X-1. Along with the published results on 1E 1740.7-2942, we have completed a high-resolution spectroscopic survey of such systems. The observed X-ray spectra of LMC X-1 and LMC X-3 show no prominent discrete features, while absorption edges (Mg K and Si K) are detected in the spectrum of GRS 1758-258. The edges are likely to be of interstellar origin. In most cases, the X-ray continuum can be described well by models that are often adopted in low-resolution studies of black hole candidates: a multi-temperature disk spectrum plus a Comptonization component. However, the relative contribution of the two components varies greatly among different sources. For instance, only the disk component is present for LMC X-1 and GRS 1758 258, while the Comptonized component is required for other sources. We discuss general issues related to obtaining disk parameters from modeling X-ray continuum.