Abdo A.A, Ackermann M., Ajello M., Allafort A., Baldini L., Ballet J., Barbiellini G., Baring M.G, Bastieri D., Bechtol K. et al..  2011.  Insights into the High-energy $\gamma$-ray Emission of Markarian 501 from Extensive Multifrequency Observations in the Fermi Era}. The Astrophysical Journal. 727:129.

Acciari V.A, Arlen T., Aune T., Beilicke M., Benbow W., Böttcher M., Boltuch D., Bradbury S.M, Buckley J.H, Bugaev V. et al..  2011.  Spectral Energy Distribution of Markarian 501: Quiescent State Versus Extreme Outburst. The Astrophysical Journal. 729:2.