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

TL;DR

This study analyzes 138 weeks of Fermi gamma-ray data of blazar 3C 454.3, revealing how gamma-ray opacity variations constrain the emission region’s location near the broad-line region boundary, supporting jet acceleration close to the black hole.

Contribution

It provides new insights into the gamma-ray emission zone’s proximity to the black hole by analyzing spectral variations and absorption features over time.

Findings

Gamma-ray spectra are well described by a lognormal distribution with absorption breaks.

The peak energy shows a power-law dependence on flux.

An anti-correlation exists between He II LyC column density and flux.

Abstract

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. This implies that the gamma-ray emission zone lies close to the boundary of the high-ionization part of the broad-line region and moves away from the black hole when the flux increases. Identification of the gamma-ray production with the relativistic jet, implies that the jet is already accelerated at sub-parsec distances from the central black hole, which favours the Blandford-Znajek process as the jet launching mechanism.