During the past week, NuSTAR responded to a Director’s Discretionary Time request for a rapid follow-up of a bright stellar flare reported by the Einstein Probe Wide-field X-ray Telescope (EP-WXT) on August 2, 2025. Stellar flares are explosive releases of magnetic energy on active stars that heat plasma and accelerate particles, producing bright, rapidly evolving X-ray emission. A growing number of studies have found that some flares are accompanied by brief episodes of strong X-ray absorption, when cool, dense ejecta lifted from the low corona crosses the line of sight and temporarily boosts photoelectric opacity. Such ejecta can naturally arise if a coronal mass ejection (CME) is launched: clumpy prominences and filament material in the CME front and wake may partially cover the X-ray source, suppressing low-energy photons while higher-energy X-rays continue to escape, and producing sharp spectral hardening and energy-dependent dips. NuSTAR is ideal for probing transient X-ray absorption and searching for hard-X-ray signatures of CME-driven obscuration. By tracking spectral hardness and effective column density through the flare decay, the observations will constrain the geometry and dynamics of the absorber and evaluate the CME interpretation. Combined with EP discovery data and ongoing monitoring, this NuSTAR dataset is expected to place stringent hard-X-ray constraints on this event and provide a template for rapid characterization of similar nearby stellar flares.

Authors: Yifan Hu (Imperial College London, UK)