XRISM/Resolve Reveals the Profiles of Neutral Iron Emission Lines in an Outburst X-Ray Spectrum of the Be/X-Ray Binary Pulsar 1A 1118-61

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Abstract We report on the high-resolution X-ray spectroscopy of the Be/X-ray binary pulsar (BeXBP) 1A 1118−61 during the outburst in 2026 January–February with X-Ray Imaging and Spectroscopy Mission (XRISM)/Resolve. We have successfully resolved the structure of neutral Fe K α emission lines (including K α 1 and K α 2 ) and found that they consist of two distinct components with line widths of σ ∼ 2 eV (∼100 km s −1 ) and σ ∼ 40 eV (∼2000 km s −1 ). The Resolve data also allow us to perform pulse-phase-resolved spectroscopy. For the narrow ( σ ∼ 2 eV) component, the line flux and equivalent width vary with the pulse phase, whereas its centroid energy and width do not. In contrast, for the broad ( σ ∼ 40 eV) line component, the centroid energy and width show pulse-phase dependence, while the line flux and equivalent width do not show significant changes. These results suggest that the broad Fe K α emission likely originates from the inner region of the accretion disk truncated near the magnetospheric radius, while the narrow Fe K α emission may arise from the Be star and/or its surrounding circumstellar disk.

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