Self-confinement of relativistic pair beams in magnetized interstellar plasmas: the case of pulsar X-ray filaments

Abstract

The observation of filamentary X-ray structures near bow-shock pulsar wind nebulae (PWNe) -- such as the Guitar, Lighthouse, and PSR J2030$+$4415 nebulae -- and of slow-diffusion regions around pulsars like Geminga, Monogem, and PSR J0622$+$3749, challenges the standard picture of cosmic-ray transport in the interstellar medium, implying a diffusion coefficient two orders of magnitude smaller than the Galactic average. The suppressed diffusion can be attributed to self-generated magnetic turbulence, driven -- via the non-resonant streaming instability -- by electron-positron pairs escaping the PWNe. This instability requires a net current, yet the beam of escaping pairs is expected to be charge-neutral. We show that a charge-neutral pair beam propagating through an electron-proton plasma can spontaneously generate a net current. Using fully kinetic two-dimensional particle-in-cell simulations with realistic mass ratio, we find that beam electrons get focused into self-generated magnetic filaments produced by the nonlinear evolution of the Weibel instability, while beam positrons remain unconfined. We show that in three-dimensional simulations the resulting net (positron) current drives the non-resonant streaming instability, further amplifying the magnetic field. This mechanism provides a pathway for the onset of charge asymmetries in initially charge-neutral pair beams and for the growth of magnetic fluctuations that efficiently scatter the beam particles, with implications for the formation of X-ray filaments and, potentially, for particle self-confinement in TeV halos around PWNe.