Energy-Dependent Lags in the 2006 Flare of PKS 2155-304: A Null Result Converted to Bounds

The giant flare of the blazar PKS 2155-304 recorded by H.E.S.S. on 28 July 2006 rose and fell on timescales of a few minutes, and a search for energy-dependent lags between its lower- and higher-energy emission found none. Converting a conservative three-hundred-second lag limit, with effective band energies of 0.4 and 2 TeV at redshift 0.116, into propagation bounds gives a linear scale above 2.911e+17 GeV and a quadratic scale above 3.331e+10 GeV. The full likelihood analysis published by the collaboration reached a stronger linear figure, as expected for a method that uses the flare's whole light curve rather than a single lag number; the simpler conversion here brackets it from below and shows what the null result alone guarantees. The framework examined here predicts exactly this null — no vacuum lag at any energy — and the flare is consistent with that prediction. Computed from published data; not a new measurement.