Abstract
Context. The origin of ultra-rapid flares of very high-energy radiation from active galactic nuclei remains elusive. Magnetospheric processes, occurring in the close vicinity of the central black hole, could account for these flares. Aims. Our aim is to bridge the gap between simulations and observations by synthesizing gamma-ray light curves in order to characterize the activity of a black hole magnetosphere, using kinetic simulations. Methods. We performed global axisymmetric 2D general-relativistic particle-in-cell simulations of a Kerr black hole magnetosphere. We included a self-consistent treatment of radiative processes and plasma supply, as well as a realistic magnetic configuration, with a large-scale equatorial current sheet. We coupled our particle-in-cell code with a ray-tracing algorithm in order to produce synthetic light curves. Results. These simulations show a highly dynamic magnetosphere, as well as very efficient dissipation of the magnetic energy. An external supply of magnetic flux is found to maintain the magnetosphere in a dynamic state, otherwise the magnetosphere settles in a quasi-steady Wald-like configuration. The dissipated energy is mostly converted to gamma-ray photons. The light curves at low viewing angle (face-on) mainly trace the spark gap activity and exhibit high variability. On the other hand, no significant variability is found at high viewing angle (edge-on), where the main contribution comes from the reconnecting current sheet. Conclusions. We observe that black hole magnetospheres with a current sheet are characterized by a very high radiative efficiency. The typical amplitude of the flares in our simulations is lower than is detected in active galactic nuclei. These flares could result from the variation in parameters external to the black hole.
Highlights
Ground-based Cherenkov telescopes have shown that active galactic nuclei (AGN) can be highly variable sources of very high-energy (VHE) emission (>100 GeV) (Aharonian et al 2007; Albert et al 2007; Aleksicet al. 2014)
This is expected since the monopole simulations show little structure in the orthoradial direction, and photons are mainly emitted radially by particles flowing along the magnetic field lines
This is consistent with the dissipation rate of electromagnetic energy measured in C20, and confirms the hypothesis that the dissipated Poynting flux is mainly transferred to photons below the pair-creation threshold
Summary
Ground-based Cherenkov telescopes have shown that active galactic nuclei (AGN) can be highly variable sources of very high-energy (VHE) emission (>100 GeV) (Aharonian et al 2007; Albert et al 2007; Aleksicet al. 2014). Variability timescales can be shorter than the horizon light-crossing time tg = rg/c, where rg is the gravitational radius of the central supermassive black hole This constrains emission models, as the size of the emitting region must be on the order of rg by virtue of causality. TeV γ-ray flares were detected from the nuclei of the radio galaxies M 87 (Aharonian et al 2006; Aliu et al 2012) and Centaurus A (Aharonian et al 2009) for instance, these galaxies having jets misaligned with our line of sight by more than 15◦ This suggests that VHE flares are a widespread feature in AGN. M 87 is of paramount importance, and has attracted considerable attention because it is close enough that its nucleus can be resolved
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