Fermi Gamma-ray Space Telescope

The Spectra and Luminosity Function of Gamma-Ray Millisecond Pulsars

Tim Linden
Ilias Cholis, Dan Hooper

Abstract:

Observations by the Fermi-LAT have uncovered a substantial population of gamma-ray bright millisecond pulsars (MSPs) in our galaxy. Using 5.5 years of Fermi-LAT data, we measure the average spectrum and luminosity of recycled pulsars, producing the most comprehensive analysis of the time-averaged emission from this population. Notably, we use these results to produce the first direct measurement of the gamma-ray luminosity function of MSPs. Additionally, we compare the average MSP characteristics with the population of observed globular clusters, which are expected to be gamma-ray bright due to their significant MSP population. We find that the average spectrum of observed globular clusters is somewhat softer than that of observed MSPs at low energies, possibly indicating that low-luminosity MSPs have a softer spectrum than detected systems, or that another emission mechanism may exist in globular clusters. Finally, we compare the MSP luminosity function with the luminosities of observed globular clusters in order to place strong constraints on contribution of sub-threshold MSPs. Using this result, we place strong constraints on the diffuse emission from undetected MSPs, and show that they are unable to account for the luminosity of the gamma-ray signal observed in the galactic center region.