Discovery of a flux-related change of the cyclotron line energy in Hercules X-1 R. Staubert, N. I. Shakura, K. Postnov, J. Wilms, R. E. Rothschild, W. Coburn, L. Rodina and D. Klochkov A&A, 465 2 (2007) L25-L28 Published online: 20 February 2007 DOI: 10.1051/0004-6361:20077098