Emission and absorption lines in the photospheric radius expansion bursts of 4U 1820–30

¹ Key Laboratory of Stars and Interstellar Medium, Xiangtan University, Xiangtan, 411105 Hunan, P.R. China
² Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing, 100049 China
³ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
⁴ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

^⋆ Corresponding authors; lizhaosheng@xtu.edu.cn; panyy@xtu.edu.cn

Received: 25 November 2024
Accepted: 10 March 2025

Abstract

We analyze the emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820–30, observed with the Neutron Star Interior Composition Explorer (NICER). Using Monte Carlo simulations to estimate the significance, we identified a 1 kiloelectron volt (keV) emission line from 14 bursts, a 3 keV absorption line from 12 bursts, and a 1.6 keV absorption line from one burst. By coadding the burst spectra at the maximum radius phase, we detected a 1.034 keV emission line with significance of 14.2σ, along with absorption lines at 1.64 and 3 keV, with significances of 10.8σ and 11.7σ, respectively. The observed energy shifts are consistent with the prediction from the burst-driven wind model, indicating that all three spectral features are produced by the PRE wind. An analysis of the ratios between the emission and absorption line energies suggests that the 1 keV feature is a superposition of several narrower Fe L-shell lines. To evaluate the scientific capabilities of the Hot Universe Baryon Surveyor (HUBS), we simulated mock observations of multiple narrow lines near 1 keV. These results demonstrate that HUBS is well suited for detailed studies of the 1 keV emission line during bursts, offering significant potential to advance our understanding of these phenomena.