Constraints on the Emission Geometries of Gamma-ray Millisecond Pulsars Observed with the Fermi Large Area Telescope

TL;DR

This paper models gamma-ray emission geometries of millisecond pulsars using Fermi LAT data, revealing insights into their magnetic inclination, viewing angles, and implications for pulsar evolution and background contributions.

Contribution

Introduces a Markov chain Monte Carlo method to fit gamma-ray and radio emission models to MSP data, providing new constraints on their emission geometries and magnetic orientations.

Findings

Best-fit viewing angles follow a uniform distribution.

Magnetic inclination angles are evenly distributed, supporting spin-up via accretion.

Radio emission likely occurs nearer the light cylinder.

Abstract

Millisecond pulsars (MSPs) have been established as a class of high-energy ($\geq$0.1 GeV) emitters with the Large Area Telescope (LAT) aboard the \emph{Fermi Gamma-ray Space Telescope}. Most MSP gamma-ray light curves display sharp peaks indicative of thin accelerating gaps, suggesting copious pair-creation in the open volume. MSP gamma-ray and radio light curves have been simulated using geometric outer-gap (OG), slot-gap/two-pole caustic (TPC), and pair-starved polar cap gamma-ray models and either a hollow-cone beam or altitude-limited, outer-magnetospheric gap radio model, all assuming a vacuum retarded dipolar magnetic field geometry. A Markov chain Monte Carlo maximum likelihood technique has been developed to find the best-fit model parameters for nineteen MSPs using data from the LAT and various radio observatories. The best-fit viewing angles follow a uniform, angular distribution. The distribution of magnetic inclination angles favors all angles equally, contrary to analyses of non-recycled pulsars, which supports the theory that MSPs have been spun-up via accretion. There are suggestions that the radio emission should occur nearer the light cylinder. These results have implications for MSP population simulations and for addressing MSP contributions to diffuse backgrounds. An implied transition in the gamma-ray luminosity versus spin-down power trend is observed but more statistics are necessary to describe it.