Very high energy outburst of Markarian 501 in May 2009 E. Aliu, S. Archambault, A. Archer, T. Arlen, T. Aune, A. Barnacka, B. Behera, M. Beilicke, W. Benbow, K. Berger et al. (102 more) A&A, 594 (2016) A76 Published online: 14 October 2016 DOI: 10.1051/0004-6361/201628744