Cyclotron line energy in Hercules X-1: stable after the decay R. Staubert, L. Ducci, L. Ji, F. Fürst, J. Wilms, R. E. Rothschild, K. Pottschmidt, M. Brumback and F. Harrison A&A, 642 (2020) A196 Published online: 20 October 2020 DOI: 10.1051/0004-6361/202038855