Emission and absorption lines in the photospheric radius expansion bursts of 4U 1820–30 Wenhui Yu, Zhaosheng Li, Yuanyue Pan, Xuejuan Yang, Yupeng Chen, Shu Zhang and Maurizio Falanga A&A, 696 (2025) A139 Published online: 09 April 2025 DOI: 10.1051/0004-6361/202453142