Active Galactic Nuclei (AGN) are among the most luminous objects in the universe. They are a particular class of galaxies which host a supermassive black hole in their central region. The black hole accretes surrounding material, which causes two opposite relativistic jets to form. The electromagnetic radiation from these jets ranges from radio emission up to very-high-energy (VHE) gamma-ray emission.

According to the angle of sight of the observer, AGN are organized into different classes. Blazars, AGN with the relavitivistic jet pointing towards the earth, are the most numerous class of VHE gamma-ray emitters. They often exhibit variability, periods of quiescence alternate with high activity states, in which the flux increases in very short timescales.

The origin of their VHE emission is still under debate. There are two big classes of emission models, leptonic and hadronic ones. The former explains the high-energy component as inverse Compton scattering of low energetic photons by relativistic electrons, while according to the latter the VHE emission is synchrotron radiation of relativistic protons and, in some cases, radiation coming from the cascades generated by proton-gamma collisions.

Thanks to the sensitivity of ca. 7 % CU (CU= crab nebula flux) in 50 hours down to energies of 60-100 GeV, the MAGIC system is particularly suited for the study of sources whose emission is concentrated at the lower end of the VHE regime. While this is an overlapping regime with Fermi-LAT, the much larger effective area of MAGIC translates in a better detector for fast varying source. The MAGIC collaboration conducts indeed a very successful monitoring and target-of-opportunity campaign aimed at the study of such sources.