Variable VHE gamma-ray emission from non-blazar AGNs

Context. The observation of rapidly variable very high energy (VHE) gamma-rays from non-aligned active galactic nuclei (AGNs), as reported from M87, proves challenging for conventional theoretical acceleration and emission models. Aims. Motivated by recent work on pulsar-type particle acceleration in M87 (Neronov & Aharonian 2007), we re-examine the centrifugal acceleration of particles by rotating jet magnetospheres in the vicinity of accreting supermassive black hole systems and analyze the energy constraints imposed for highly underluminous systems. Methods. The maximum Lorentz factor for centrifugally accelerated electrons in the presence of inverse Compton losses, and the associated characteristic variability time scale, are determined. Applications are presented for conditions expected to be present in the radio galaxy M87, assuming accretion onto the central black hole to occur in an advection-dominated (ADAF) mode. Results. We show that for a highly underluminous source like M87, centrifugally accelerated electrons may reach Lorentz factors up to γ ∼ (107 − 108), allowing inverse Compton (Thomson) upscattering of sub-mm disk photons to the TeV regime. Upscattering of Comptonized disk photons results in a flat TeV spectrum Lν ∝ ν−αc with spectral index αc ≃ 1.2. The characteristic variability time scale is of the order rL/c, which in the case of M87 corresponds to ≃ 1.7 d for a typical light cylinder radius of rL ≃ 5 rs. Conclusions. Centrifugal acceleration could provide a natural explanation for the challenging VHE emission features in M87. Our results suggest that some advection-dominated accreting (non-blazar) AGNs could well be observable VHE emitting sources.