Strong Support for the Millisecond Pulsar Origin of the Galactic Center GeV Excess

Using $\gamma$-ray data from the Fermi Large Area Telescope, various groups have identified a clear excess emission in the Inner Galaxy, at energies around a few GeV. This excess resembles remarkably well a signal from dark-matter annihilation. One of the most compelling astrophysical interpretations is that the excess is caused by the combined effect of a previously undetected population of dim $\gamma$-ray sources. Because of their spectral similarity, the best candidates are millisecond pulsars. Here, we search for this hypothetical source population, using a novel approach based on wavelet decomposition of the $\gamma$-ray sky and the statistics of Gaussian random fields. Using almost seven years of Fermi-LAT data, we detect a clustering of photons as predicted for the hypothetical population of millisecond pulsar, with a statistical significance of $10.0\sigma$. For plausible values of the luminosity function, this population explains 100% of the observed excess emission. We argue that other extragalactic or Galactic sources, a mismodeling of Galactic diffuse emission, or the thick-disk population of pulsars are unlikely to account for this observation.

Figure 1

SNR of the wavelet transform of $\gamma$ rays with energies in the range 1–4 GeV, $\mathcal{S}(\mathrm{\Omega })$. The black circles show the position of wavelet peaks with $\mathcal{S}\ge 2$; the red circles show the position of third Fermi-LAT catalog (3FGL) sources. In both cases, the circle area scales with the significance of the source detection in that energy range. The dashed lines indicate the regions that we use for the binned likelihood analysis, where latitudes $|b|<2°$ are excluded because of the strong emission from the Galactic disk. The subset of 3FGL sources that remains unmasked in our analysis is indicated by the green crosses.

Figure 2

Histogram of observed peaks in $\mathcal{S}(\mathrm{\Omega })$ in bins of projected radial distance from the GC and SNR values (black dots with statistical error bars). We show the expectation value for the case of a negligible CSP as blue bars, whereas the expectation values for the best fit are shown in red.

Figure 3

Constraints on the maximum $\gamma$-ray luminosity of the CSP, $L_{\max }$, and the population averaged intensity at $b=\pm 5°$, $\ell =0°$, and 2 GeV, ${\mathrm{\Phi }}_5$, as derived from our wavelet analysis. We show 68.7%, 95.4%, and 99.7% C.L. contours. We also indicate the values of ${\mathrm{\Phi }}_5$ where the source population can explain 100% of the GCE (horizontal gray band, 95.4% C.L.) and as vertical orange lines the luminosity of the brightest observed nearby MSPs.