Axion-like particles and emission of very high energy gamma rays in blazars
2012-12-12 14:00 Brera - Cupola Fiore
The discovery of very high energy (E>100 GeV) emission from flat spectrum radio quasars went as a surprise. In fact, the nuclear environment of these blazars, characterized by prominent broad emission lines in the optical spectra (produced in the so-called broad line region), is expected to be highly opaque to gamma rays above ~20 GeV. Although a possible explanation is that the emission occurred well outside the inner blazar region (distances larger than 0.1-1 pc), the very rapid VHE flux variation (~ten minutes) displayed by the source PKS 1222+216 is very difficult to reproduce in this view. We present an alternative model based on the assumption that the photon escape is allowed by photon-ALP conversions before the BLR and subsequent re-conversion in the host galaxy. An acceptable shape of the intrinsic spectrum can be obtained for values of the ALP-photon coupling of the order of 10^11 GeV, close to values testable with the upgraded photon regeneration ALPS experiment at DESY.