Long-term change in the cyclotron line energy in Hercules X-1 R. Staubert, D. Klochkov, J. Wilms, K. Postnov, N. I. Shakura, R. E. Rothschild, F. Fürst and F. A. Harrison A&A, 572 (2014) A119 Published online: 09 December 2014 DOI: 10.1051/0004-6361/201424203