“LEADING BLOB” MODEL IN A STOCHASTIC ACCELERATION SCENARIO: THE CASE OF THE 2009 FLARE OF Mkn 501

Evidence for very hard, intrinsic Delta *g-ray source spectra, as inferred after correction for absorption in the extragalactic background light (EBL), has interesting implications for the acceleration and radiation mechanisms acting in blazars. A key issue so far has been the dependence of the hardness of the Delta *g-ray spectrum on different existing EBL models. The recent Fermi observations of Mkn 501 now provide additional evidence for the presence of hard intrinsic Delta *g-ray spectra independent of EBL uncertainties. Relativistic Maxwellian-type electron energy distributions that are formed in stochastic acceleration scenarios offer a plausible interpretation for such hard source spectra. Here, we show that the combined emission from different components with Maxwellian-type distributions could in principle also account for much softer and broader power-law-like emission spectra. We introduce a 'leading blob' scenario, applicable to active flaring episodes, when one (or a few) of these components become distinct over the 'background' emission, producing hard spectral features and/or hardening of the observed spectra. We show that this model can explain the peculiar high-energy characteristics of Mkn 501 in 2009, with evidence for flaring activity and strong spectral hardening at the highest Delta *g-ray energies.