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.

Humphrey A., Villar-Martín M., Sánchez S.F, Martínez-Sansigre A., Delgado R.G, Pérez E., Tadhunter C., Pérez-Torres M.A.  2010.  Integral-field spectroscopy of type II QSOs at z = 0.3-0.4. Monthly Notices of the Royal Astronomical Society. 408:L1-L5.