Fermi Gamma-ray Space Telescope

Effects of two-photon pair production on reconnection in the outer magnetosphere of gamma-ray pulsars

Hayk Hakobyan
(Alexander Philippov, Anatoly Spitkovsky)


Gamma-ray pulsars are thought to produce high energy radiation in the outer magnetosphere close to the light cylinder via synchrotron radiation from nonthermal particles sustained in the current sheet. Magnetic reconnection near the Y-point is an efficient mechanism for tapping the magnetic field energy and accelerating particles that can later radiate synchrotron photons. The presence of a substantial density of high energy photons (>MeV) close to the current sheet can make two-photon pair production an efficient source of additional plasma loading in the outer magnetosphere. We present the results of novel particle-in-cell simulations of relativistic reconnection with self-consistent pair production that track both the radiated synchrotron photons and the created pairs. We study the effects of additional pair loading on the two-dimensional reconnecting current sheet. Synchrotron radiation from accelerated particles in the current sheet produces hot secondary pairs in the upstream which are later advected into the current sheet where they are reaccelerated and produce more photons. We study how the inflow of the secondary plasma, with multiplicities up to a several hundred, reduces the effective magnetization of the current sheet and driving the system to a self-regulated regime. Our results offer an explanation for the weak dependence observed by Fermi satellite of the gamma-ray cutoff in pulsars on the magnetic field at the light cylinder.