M 87 as a misaligned synchrotron-proton blazar

¹ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- & Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany e-mail: afm@tp4.rub.de
² Department of Physics and Mathematical Physics, The University of Adelaide, Adelaide SA 5005, Australia e-mail: [rprother; adonea]@physics.adelaide.edu.au

Corresponding author: A. Reimer, afm@tp4.ruhr-uni-bochum.de

Received: 27 August 2003
Accepted: 3 February 2004

Abstract

The giant radio galaxy M 87 is usually classified as a Fanaroff-Riley class I source, suggesting that M 87 is a mis-aligned BL Lac object. Its unresolved nuclear region emits strong non-thermal emission from radio to X-rays which has been interpreted as synchrotron radiation. In an earlier paper we predicted M 87 as a source of detectable gamma ray emission in the context of the hadronic Synchrotron-Proton Blazar (SPB) model. The subsequent tentative detection of  TeV energy photons by the HEGRA-telescope array would, if confirmed, make it the first radio galaxy to be detected at TeV-energies. We discuss the emission from the unresolved nuclear region of M 87 in the context of the SPB model, and give examples of possible model representations of its non-simultaneous spectral energy distribution. The low-energy component can be explained as synchrotron radiation by a primary relativistic electron population that is injected together with energetic protons into a highly magnetized emission region. We find that the γ-ray power output is dominated either by / synchrotron or proton synchrotron radiation depending on whether the primary electron synchrotron component peaks at low or high energies, respectively. The predicted γ-ray luminosity peaks at ~100 GeV at a level comparable to that of the low-energy hump, and this makes M 87 a promising candidate source for the newly-commissioned high-sensitivity low-threshold Cherenkov telescopes H.E.S.S., VERITAS, MAGIC and CANGAROO III. Because of its proximity, the high-energy spectrum of M 87 is unaffected by absorption in the cosmic infrared (IR) background radiation field, and could therefore serve as a template spectrum for the corresponding class of blazar if corrected for mis-alignment effects. This could significantly push efforts to constrain the cosmic IR radiation field through observation of more distant TeV-blazars, and could have a strong impact on blazar emission models. If M 87 is a mis-aligned BL-Lac object and produces TeV-photons as recently detected by the HEGRA-array, in the context of the SPB model it must also be an efficient proton accelerator.