High variability in Vela X-1: giant flares and off states I. Kreykenbohm, J. Wilms, P. Kretschmar, J. M. Torrejón, K. Pottschmidt, M. Hanke, A. Santangelo, C. Ferrigno and R. Staubert A&A, 492 2 (2008) 511-525 Published online: 27 October 2008 DOI: 10.1051/0004-6361:200809956