The role of Bethe-Heitler pair production in reconnection-driven flares in M87*

Abstract

Rapid TeV flares have been observed from the core of the active galaxy M87. These have been attributed to inverse Compton scattering of disk photons by electrons and positrons accelerated in transient reconnection layers formed in baryon-poor regions of the magnetosphere of the central black hole, M87*. It was previously shown that even a small number of protons accelerated in the same layers can lead to bright GeV proton-synchrotron flares, if protons receive $\gtrsim20\%$ of the dissipated power for reconnecting fields of $\sim$100 G. We aim to investigate the role of Bethe-Heitler pair production in the emission of reconnection-driven flares from M87* in this physical regime. We perform numerical calculations that incorporate inelastic collisions between relativistic protons and photons, as well as photon-photon pair production, and compute the non-thermal radiation from the layer. The numerical calculations are also supported by analytical estimates. We find that disk photons act as targets for Bethe-Heitler pair production. The resulting pairs emit very high-energy synchrotron photons ($\gtrsim$0.1 TeV), which are subsequently attenuated by the disk photon field, leading to further pair production. The synchrotron emission of these secondary pairs produces soft photons, as part of an electromagnetic cascade, enhancing pion production and photon-photon attenuation down to tens-of-GeV energies.

Figures (6)

• Figure 1: Schematic illustration of interactions among charged particles and photons in a magnetospheric current layer. Solid arrows denote proton synchrotron radiation producing high-energy (HE) photons (orange shaded box), Bethe-Heitler pair production (p$\gamma \rightarrow e^{-}e^{+}$), and photopion production (p$\gamma \rightarrow \pi^{\pm}, \pi^0$). The latter two processes (indicated with blue arrows) rely only on ambient disk photons (blue shaded box), and lead respectively to the generation of very-high-energy (VHE) and ultra-high-energy (UHE) photons (yellow and pink shaded boxes respectively). Dotted lines connect photon populations that interact via photon-photon ($\gamma \gamma$) pair creation, producing secondary pairs (gray shaded triangle). Low-energy (LE) photons (green shaded box) that are emitted by secondary pairs through synchrotron provide an additional target for photopion interactions. Thick arrows indicate the dominant interaction channels.
• Figure 2: Spectral energy distribution of photons (black lines) and neutrinos (dark red lines) produced by relativistic protons accelerated in a magnetospheric current sheet in M87. Thick lines show the escaping photon and neutrino radiation when all processes are taken into account. The magenta-shaded region indicates the range of proton synchrotron spectra presented in https://ui.adsabs.harvard.edu/abs/2025ApJ...995L..73H. Energy ranges discussed in text are colored following the coloring scheme in Fig. \ref{['fig:sketch']}. Grey and indigo markers show multi-wavelength observations of M87* in 2017 and 2018 at a low and flaring state respectively 2021ApJ...911L..11E2024AA...692A.140A. Overplotted with grey lines are Chandra X-ray observations from 2007 to 2019 2023RAA....23f5018C.
• Figure 3: Spectral energy distribution of photons (left) and neutrinos of all flavors (right) produced by relativistic protons accelerated in a magnetospheric current sheet in M87*. Each model is calculated for the same parameters as those of the baseline model except for one parameter that is varied, as shown in the inset legend. Models that differ significantly from the baseline model are plotted with thin lines.
• Figure 4: Peak neutrino luminosity plotted against $\sigma_{\rm p}$ (left panel) and $\eta_{\rm p}$ (right panel). Dashed black lines show simple analytical scalings that do not account for non-linear effects.
• Figure 5: Ratio of optical depths for isotropic and perpendicular only collisions.