A model for delayed emission in a very - high energy gamma - ray flare in Markarian 501

Context. Recently, the MAGIC collaboration reported evidence for a delay in the arrival times of photons of different energies during a γ-ray flare from the blazar Markarian 501 on 2005 July 9. Aims. We aim at describing the observed delayed high-energy emission. Methods. We apply a homogeneous synchrotron self-Compton (SSC) model under the assumption that the blob containing relativistic electrons was observed in its acceleration phase. Results. This modified SSC model predicts the appearance of a γ-ray flare first at lower energies and subsequently at higher energies. Conclusions. Based on the reported time delay of ∼ 240 s between the flare observed at ∼ 190 GeV and 2.7 TeV, we predict a delay on the order of 1 h if observed between 10 GeV and 100 GeV. Such delay timescales can be tested in the future by simultaneous flare observations with the Gamma Ray Large Area Space Telescope (GLAST) and Cherenkov telescopes.