A synchrotron self-Compton scenario for the very high energy gamma-ray emission of the radiogalaxy M87

TL;DR

This paper proposes a new multi-blob synchrotron self-Compton model to explain the very high energy gamma-ray emission and rapid variability of M87, accounting for observations at large viewing angles and moderate Lorentz factors.

Contribution

It introduces a novel multi-blob SSC model for misaligned jets, specifically tailored for M87, incorporating plasma blobs beyond the Alfven surface at about 100 r_g from the black hole.

Findings

Model explains TeV spectra and variability of M87

Blob radii of about 10^{14} cm match observed variability timescales

Predictions extend to other misaligned AGN like Cen A, PKS 0521-36, and 3C 273

Abstract

M87 is the first extragalactic source detected in the TeV range that is not a blazar. With the increasing performances of ground-based Cherenkov telescopes, we can now probe the variability in the gamma-ray flux at small timescales, thus putting strong constraints on the size of the emitting zone. A modification of standard emission models of TeV blazars appears necessary to account for the gamma-ray observations despite this misalignment. We explain TeV gamma-ray spectra and fast variability of M87 by invoking an emission zone close to the central supermassive black hole, which is filled with several plasma blobs moving in the large opening angle of the jet formation zone. We develop a new multi-blob synchrotron self-Compton (SSC) model with emitting blobs beyond the Alfven surface in the jet, at a distance of about 100 r_g from the central engine. This model is explicitly adapted to deal with large viewing angles and moderate values of the Lorentz factor inferred from (general relativistic) magnetohydrodynamic models of jet formation. This scenario can account for the recent gamma-ray observations of M87 made by the High Energy Stereoscopic System (H.E.S.S.) telescope array. We find individual blob radii of about 10^{14} cm, which is compatible with the variability on timescales of days recently reported by the H.E.S.S. collaboration and is of the order of the black hole gravitational radius. Predictions of the very high energy emission for three other sources with extended optical or X-ray jet which could be misaligned blazars still with moderate beaming are presented for one Seyfert 2 radiogalaxy, namely Cen A, one peculiar BL Lac, PKS 0521-36, and one quasar, 3C 273.