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.