Millisecond Pulsar Kicks Cause Difficulties in Explaining the Galactic Center Gamma-Ray Excess

TL;DR

This study uses N-body simulations to evaluate whether millisecond pulsars can explain the Galactic Center gamma-ray excess, finding that pulsars alone cannot fully account for the observed concentrated emission.

Contribution

The paper presents detailed dynamical simulations of millisecond pulsars with natal kicks, assessing their spatial distribution and gamma-ray emission contribution in the Galactic Center.

Findings

Pulsar models without natal kicks better match the central gamma-ray brightness.

Simulations including globular cluster deposits can reproduce some observed profiles.

Pure pulsar models cannot fully explain the gamma-ray excess concentration.

Abstract

The unexplained excess gamma-ray emission from the Milky Way's Galactic Center has puzzled astronomers for nearly a decade. Two theories strive to explain the origin of this excess: self-annihilating dark matter particles or an unresolved population of radio millisecond pulsars. We examine the plausibility of a pulsar origin for the GeV excess using N-body simulations. We simulated millisecond pulsars in a realistic dynamical environment: (i) pulsars were born from the known stellar mass components of our Galaxy; (ii) pulsars were given natal velocity kicks as empirically observed from two different studies (or, for comparison, without kicks); (iii) pulsars were evolved in a Galactic gravitational potential consistent with observations. Multiple populations of pulsars (with different velocity kicks) were simulated over 1 Gyr. With final spatial distributions of pulsars, we constructed synthetic gamma-ray surface brightness profiles. From comparisons with published Fermi-LAT surface brightness profiles, our pulsar simulations cannot reproduce the concentrated emission in the central degrees of the Bulge, though models without natal velocity kicks approach the data. We considered additive combinations of our (primordial MSP) simulations with models where pulsars are deposited from destroyed globular clusters in the Bulge, and a simple model for pulsars produced in the nuclear star cluster. We can reasonably reproduce the measured central gamma-ray surface brightness distribution of Horiuchi and collaborators using several combinations of these models, but we cannot reproduce the measured distribution of Di Mauro with any combination of models. Our fits provide constraints on potential pathways to explain the gamma-ray excess using MSPs.