New insights with XRISM and Cloudy: A novel column density diagnostic

Aims. We present a simple yet powerful column density diagnostic for plasmas enabled by X-ray microcalorimeter observations. Methods. With the recent developments of the spectral simulation code C LOUDY , inspired by the high spectral resolution of the X-Ray Imaging and Spectroscopy Mission (XRISM) and the Advanced Telescope for High Energy Astrophysics (Athena), we make predictions for the intensity ratio of the resolved fine-structure lines Ly α 1 and Ly α 2 of H-like ions. Results. We show that this ratio can be observationally constrained and used as a plasma column density indicator. We demonstrate this with an XRISM observation of the high-mass X-ray binary Centaurus X-3. Conclusions. This diagnostic is useful for a wide range of X-ray emitting plasmas that are either collisionally or radiatively ionized.