Millisecond pulsars cannot account for the inner Galaxy’s GeV excess

Abstract

Using data from the Fermi Gamma-Ray Space Telescope, a spatially extended component of gamma rays has been identified from the direction of the Galactic center, peaking at energies of $\ensuremath{\sim}2--3\text{ }\text{ }\mathrm{GeV}$. More recently, it has been shown that this signal is not confined to the innermost hundreds of parsecs of the Galaxy, but instead extends to at least $\ensuremath{\sim}3\text{ }\text{ }\mathrm{kpc}$ from the Galactic center. While the spectrum, intensity, and angular distribution of this signal is in good agreement with predictions from annihilating dark matter, it has also been suggested that a population of unresolved millisecond pulsars could be responsible for this excess GeV emission from the inner Galaxy. In this paper, we consider this later possibility in detail. Comparing the observed spectral shape of the inner Galaxy's GeV excess to the spectrum measured from 37 millisecond pulsars by Fermi, we find that these sources exhibit a spectral shape that is much too soft at sub-GeV energies to accommodate this signal. We also construct population models to describe the spatial distribution and luminosity function of the Milky Way's millisecond pulsars. After taking into account constraints from the observed distribution of Fermi sources (including both sources known to be millisecond pulsars, and unidentified sources which could be pulsars), we find that millisecond pulsars can account for no more than $\ensuremath{\sim}10%$ of the inner Galaxy's GeV excess. Each of these arguments strongly disfavor millisecond pulsars as the source of this signal.