Absorption Lines in Photospheric Radius Expansion Bursts from 4U 1820-303
A NICER spectral study confirming and characterizing the temporal evolution of significant (>99.9%) absorption lines, including a 2.97 keV feature, across twelve type I (thermonuclear) photospheric radius expansion bursts from the ultracompact neutron-star binary 4U 1820-303. Using accretion-enhanced continuum models, SPEX plasma codes, and Monte Carlo significance testing over a temperature-velocity grid, the analysis does not reproduce the previously reported correlation between line energy and inferred blackbody radius, and favors a nearly rest-frame photo-/collisionally-ionized gas as the line origin.
Astronomy & Astrophysics manuscript no. aanda ©ESO 2025 January 6, 2025 Line detections in
A NICER X-ray spectroscopic study of type I (thermonuclear) photospheric radius expansion (PRE) bursts from the ultracompact neutron-star low-mass X-ray binary 4U 1820-303. The authors re-analyse fou…