Particle Acceleration Zones above Pulsar Polar Caps: Electron and Positron Pair Formation Fronts

Abstract

We investigate self-consistent particle acceleration near a pulsar polar cap (PC) by the electrostatic field due to the effect of inertial frame dragging. Test particles gain energy from the electric field parallel to the open magnetic field lines and lose energy by both curvature radiation (CR) and resonant and non-resonant inverse Compton scattering (ICS) with soft thermal X-rays from the neutron star (NS) surface. Gamma-rays radiated by electrons accelerated from the stellar surface produce pairs in the strong magnetic field, which screen the electric field beyond a pair formation front (PFF). Some of the created positrons can be accelerated back toward the surface and produce γ-rays and pairs that create another PFF above the surface. We find that ICS photons control PFF formation near the surface, but due to the different angles at which the electron and positron scatter the soft photons, positron initiated cascades develop above the surface and screen the accelerating electric field. Stable acceleration from the NS surface is therefore not possible in the presence of dominant ICS energy losses. However, we find that stable acceleration zones may occur at some distance above the surface, where CR dominates the electron and positron energy losses, and there is up-down symmetry between the electron and positron PFFs. We examine the dependence of CR-controlled acceleration zone voltage, width and height above the surface on parameters of the pulsar and its soft X-ray emission. For most pulsars, we find that acceleration will start at a height of 0.5 -1 stellar radii above the NS surface.

Classification and Ranking of Fermi Lat Gamma-Ray Sources From the 3fgl Catalog Using Machine Learning Techniques

“…Among the earliest scientific results from the LAT was the discovery of a large population of radio-quiet gamma-ray pulsars (Abdo et al 2009b;Saz Parkinson et al 2010), using a new blind-search technique developed specifically for the very long time series expected with the LAT (Atwood et al 2006). These findings confirmed some predictions that many of the unidentified EGRET sources in the Galactic plane were pulsars(e.g., Yadigaroglu & Romani 1995), favoring outer gap pulsar models (Cheng et al 1986;Romani 1996Romani , 2014, where the gamma rays are generated in the outer magnetosphere, as opposed to polar cap models, where the emission comes from closer to the neutron star surface(e.g., Harding & Muslimov 1998).…”

High energy neutrinos from fast spinning magnetars

“…Harding & Muslimov 1998, 2001Hibschman & Arons 2001), and the detail how the low-altitude potential is matched to that at higher altitudes. For example, for a vacuum gap one has ǫ ∼ 1/θ 2 d , which implies that the maximum potential is achieved at s ∼ θ d (Ruderman & Sutherland 1975).…”

“…For acceleration near the PC we extend the polar gap model for normal pulsars (e.g. Hibschman & Arons 2001;Harding & Muslimov 1998) to MSMs. Since the steady gap model may not be realistic as pair cascades are likely nonstationary and time-dependent, we emphasize the energetics of polar gap acceleration rather than a specific gap model.…”

Current Flows in Pulsar Magnetospheres

“…The vacuum inner gap model, on the one hand, suggested first by Ruderman & Sutherland (1975, hereafter RS75) depends on enough binding energy of charged particles on the pulsar surface. The space chargelimited flow model (e.g., Arons & Scharlemenn 1979;Harding & Muslimov 1998), on the other hand, is without any binding energy. An outer gap near light cylinder has also been proposed (e.g., Cheng, Ho, & Ruderman 1986), the existence of which may also reflect strong binding of particles on pulsar surface (Xu 2003a).…”