Dark Matter and Pulsar Model Constraints from Galactic Center Fermi-LAT Gamma Ray Observations

Abstract

Employing Fermi-LAT gamma ray observations, several independent groups have found excess extended gamma ray emission at the Galactic center (GC). Both, annihilating dark matter (DM) or a population of $\sim 10^3$ unresolved millisecond pulsars (MSPs) are regarded as well motivated possible explanations. However, there is significant uncertainties in the diffuse galactic background at the GC. We have performed a revaluation of these two models for the extended gamma ray source at the GC by accounting for the systematic uncertainties of the Galactic diffuse emission model. We also marginalize over point source and diffuse background parameters in the region of interest. We show that the excess emission is significantly more extended than a point source. We find that the DM (or pulsars population) signal is larger than the systematic errors and therefore proceed to determine the sectors of parameter space that provide an acceptable fit to the data. We found that a population of order a 1000 MSPs with parameters consistent with the average spectral shape of Fermi-LAT measured MSPs was able to fit the GC excess emission. For DM, we found that a pure $τ^+τ^-$ annihilation channel is not a good fit to the data. But a mixture of $τ^+τ^-$ and $b\bar{b}$ with a $\left<σv\right>$ of order the thermal relic value and a DM mass of around 20 to 60 GeV provides an adequate fit.

20 pages, 13 figures, v2: typos corrected and references added; v3 several clarification comments added, v4 typos corrected, matches version to appear in PRD, v5: erratum included, with a correction to the explanation of the estimate of the number of millisecond pulsars required, conclusions unchanged