A New Determination of the Spectra and Luminosity Function of Gamma-Ray Millisecond Pulsars

Abstract

In this article, we revisit the gamma-ray emission observed from millisecond pulsars and globular clusters. Based on 5.6 years of data from the Fermi Gamma-Ray Space Telescope, we report gamma-ray spectra for 61 millisecond pulsars, finding most to be well fit by a power-law with an exponential cutoff, producing to a spectral peak near ~1-2 GeV (in $E^2 dN/dE$ units). Additionally, while most globular clusters exhibit a similar spectral shape, we identify a few with significantly softer spectra. We also determine the gamma-ray luminosity function of millisecond pulsars using the population found in the nearby field of the Milky Way, and within the globular cluster 47 Tucanae. We find that the gamma-ray emission observed from globular clusters is dominated by a relatively small number of bright millisecond pulsars, and that low-luminosity pulsars account for only a small fraction of the total flux. Our results also suggest that the gamma-ray emission from millisecond pulsars is more isotropic and less strongly beamed than the emission at X-ray wavelengths. Furthermore, the observed distribution of apparent gamma-ray efficiencies provides support for the slot gap or the outer gap models over those in which the gamma-ray emission originates from regions close to the neutron star's magnetic poles (polar cap models).

28 pages, 21 figures, 4 tables