Unraveling the Emission Geometry of the Fermi Millisecond Pulsars

TL;DR

This paper investigates the emission geometry of nine Fermi-detected millisecond pulsars by modeling their gamma-ray and radio light curves to understand beam structures and orientations.

Contribution

It compares geometric models (OG, TPC, PSPC) for fitting MSP light curves and deduces their beam geometries and viewing angles, providing new insights into pulsar emission regions.

Findings

Most MSP light curves fit by OG and TPC models

PSR J0034-0534 best modeled by outer magnetosphere models

Radio emission modeled with a fixed-altitude conal model

Abstract

The nine millisecond pulsars (MSPs) that have now been detected by Fermi-LAT are providing an excellent opportunity to probe the emission geometry of these ancient compact objects. As they are radio-loud, one may use the relative phase lags across wavebands to obtain constraints on the orientation, size, and location of their radio and gamma-ray beams. We model the gamma-ray light curves using geometric outer gap (OG) and two-pole caustic (TPC) models, in addition to a pair-starved polar cap (PSPC) model which incorporates the full General Relativistic E-field. We find that most MSP light curves are fit by OG and TPC models, while PSPC is more appropriate for two others. The light curves of the newest discovery, PSR J0034-0534, are best modeled using outer magnetosphere OG / TPC models of limited extension for both radio and gamma-ray beams. We model the radio emission of the other eight MSPs using a fixed-altitude conal model at lower altitude. We lastly deduce values for inclination and observer angles (alpha and zeta), as well as the flux correction factor, in each case.