Description
Compton-driven interactions between ultraintense gamma-ray fluxes and background plasmas are expected to arise in various high-energy astrophysical settings. Self-consistent kinetic investigations into this problem are now accessible via particle-in-cell (PIC) simulations [1-3], which have recently uncovered a rich phenomenology of acceleration processes and plasma instabilities. Here, we report on the first PIC simulation study of such events allowing for electron-positron pair creation through photon-photon collisions (the linear Breit-Wheeler process).
First, we detail the Monte Carlo scheme incorporated in the CALDER code to describe the linear Breit-Wheeler process and benchmark it against the theory of Ref. [4]. We then focus on a reference configuration characterized by an electron-ion plasma of density $n_p = 10^{21} \; cm^{-3}$ , a photon energy of $\varepsilon_{\gamma} = 1.8 \; m_ec^2$, and a photon density profile rising up to $n_{\gamma} \sim 5\times10^7n_p$. The Compton-scattered photons rapidly accelerate most of the plasma electrons to relativistic speeds and subsequently interact with the incoming photons. This creates pairs in quantities far exceeding the initial plasma density, thereby drastically altering the system dynamics.
Due to their low inertia, positrons act to screen the charge-separation field, gaining significant energy (up to ~20× the incident photon energy) at the expense of the ions. While this mitigates ion acceleration compared to pair-free scenarios [3], pair creation enhances ion heating. To explain this, we have solved the linear electrostatic dispersion equation for the three-interacting species (electrons, positrons, protons) using the simulated particle distribution functions. We find that, without pair creation, ion heating stems from a kinetic ion acoustic instability triggered by the electrons drifting against the slower protons. Theoretical predictions match the simulated wave Fourier spectra. However, when enabling pair creation, the leading instability grows faster, and operates in an intermediate regime between the hydrodynamic Buneman and kinetic ion-acoustic instabilities. Finally, we examine how these results scale with varying photon energy and density.
References
[1] F. Del Gaudio et al., Phys. Rev. Let. 125, 265001 (2020).
[2] B. Martinez et al., J. Plasma Phys. 87, 905870313 (2021).
[3] J. C. Faure et al., Phys. Rev. E 109, 015203 (2024).
[4] R. Schlickeiser et al., Astrophys. J. 758, 101 (2012).