Synthetic gamma-ray lightcurves of Kerr black-hole magnetospheric activity from particle-in-cell simulations

TL;DR

This study uses advanced simulations to generate synthetic gamma-ray lightcurves of Kerr black-hole magnetospheres, revealing highly dynamic activity and efficient energy dissipation, which helps interpret rapid flares in active galactic nuclei.

Contribution

First comprehensive kinetic simulation coupling particle-in-cell methods with ray-tracing to produce realistic gamma-ray lightcurves of black-hole magnetospheres.

Findings

Magnetospheres are highly dynamic with efficient magnetic energy dissipation.

Lightcurves vary significantly with viewing angle, showing high variability face-on.

Simulated flare amplitudes are lower than observed in active galactic nuclei.

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: We aim to bridge the gap between simulations and observations by synthesizing gamma-ray lightcurves in order to characterize the activity of a black-hole magnetosphere, using kinetic simulations. Methods: We perform global axisymmetric two-dimensional general-relativistic particle-in-cell simulations of a Kerr black-hole magnetosphere. We include 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 couple our particle-in-cell code with a ray-tracing algorithm, in order to produce synthetic lightcurves. 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 lightcurves 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 what is detected in active galactic nuclei. Such flares could result from the variation of parameters external to the black hole