Quadratic Dispersion Limits from the 2005 TeV Flare of Markarian 501

During a flare of the blazar Markarian 501 on 28 June 2005, the MAGIC telescope found the TeV emission's higher-energy component delayed by roughly four minutes relative to its lower-energy component. Taking that lag entirely as propagation, and representing the two bands by effective energies of 0.35 and 2 TeV at the source's redshift of 0.034, bounds a linear dispersion scale at 1.079e+17 GeV and a quadratic scale at 1.967e+10 GeV. Quadratic terms are the interesting case at TeV energies, because a quadratic suppression that makes an effect invisible at MeV energies can still move TeV photons by minutes over a hundred-megaparsec path. The framework examined here states that vacuum photon speed is energy-independent, so it attributes the lag to the source; nothing in the flare forces a propagation reading, and the bounds above state how large a propagation term the lag could hide. Computed from published data; not a new measurement.