A synchrotron self-Compton scenario for the very high energy γ-ray emission of the radiogalaxy M 87

Unifying the TeV emission of blazars and other AGNs?

¹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot; 5 Place Jules Janssen, 92190 Meudon, France e-mail: jean-philippe.lenain@obspm.fr
² Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87100 Toruń, Poland

Received: 1 June 2007
Accepted: 12 October 2007

Abstract

Context.M 87 is the first extragalactic source detected in the TeV range that is not a blazar. With the increasing performances of ground-based Čerenkov telescopes, we can now probe the variability in the γ-ray flux on small timescales, thus putting strong constraints on the size of the emitting zone. The large scale jet of M 87 is misaligned with respect to the line of sight. A modification of standard emission models of TeV blazars appears necessary to account for the γ-ray observations despite this misalignment.

Aims.We explain TeV γ-ray spectra and the fast variability of M 87 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.

Methods.We develop a new multi-blob synchrotron self-Compton (SSC) model with emitting blobs set on a cap beyond the Alfvén surface in the jet, at a distance of ~100 r_g from the central engine to interpret the high energies inferred by new TeV observations. We present an SSC model that is explicitly adapted to dealing with large viewing angles and moderate values of the Lorentz factor inferred from (general relativistic) magnetohydrodynamic models of jet formation.

Results.This scenario can account for the recent γ-ray observations of M 87 made by the High Energy Stereoscopic System (HESS) telescope array. We find individual blob radii of about 10¹⁴ cm, that is compatible with the variability on timescales of days recently reported by the HESS collaboration and is close to the black hole gravitational radius. Predictions of the very high-energy emission for three other sources with extended optical or X-ray jet that 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.