Variation of the gamma-gamma opacity by the He II Lyman continuum constrains the location of the gamma-ray emission region in the blazar 3C 454.3

Understand

We study spectral properties of the brightest gamma-ray blazar 3C454.3 using 138 weeks of observations by the Fermi Gamma-ray Space Telescope (Fermi).

• We probe the behaviour of the source as a function of time at different brightness levels.
• The Fermi spectra in the GeV range can be well described by a wide underlying lognormal distribution with the photon-photon absorption breaks produced by the He II and H I Lyman recombination continua (LyC).
• We find a power-law dependence of the peak energy on flux and discover anti-correlation between the column density of the He II LyC and flux.