Strong Evidence for Gamma-ray Line Emission from the Inner Galaxy

Abstract

Using 3.7 years of \Fermi-LAT data, we examine the diffuse 80-200 GeV emission in the inner Galaxy and find a resolved gamma-ray feature at $\sim 110-140$ GeV. We model the spatial distribution of this emission with a $\sim3\degree$ FWHM Gaussian, finding a best fit position $1.5\degree$ West of the Galactic Center. Even better fits are obtained for off-center Einasto and power-law profiles, which are preferred over the null (no line) hypothesis by $6.5\sigma$ ($5.0\sigma/5.4\sigma$ after trials factor correction for one/two line case) assuming an NFW density profile centered at $(\ell, b)=(-1.5\degree,0\degree)$ with a power index $\alpha=1.2$ . The energy spectrum of this structure is consistent with a single spectral line (at energy $127.0\pm 2.0$ GeV with $\chi^2=4.48$ for 4 d.o.f.). A pair of lines at $110.8\pm 4.4$ GeV and $128.8\pm 2.7$ GeV provides a marginally better fit (with $\chi^2=1.25$ for 2 d.o.f.). The total luminosity of the structure is $(3.2\pm0.6)\times 10^{35}$ erg/s, or $(1.7\pm0.4)\times 10^{36}$ photons/sec. The energies in the two-line case are compatible with a $127.3 \pm 2.7$ GeV WIMP annihilating through $\gamma \gamma$ and $\gamma Z$ (with $\chi^2=1.67$ for 3 d.o.f.). We describe a possible change to the \Fermi\ scan strategy that would accumulate S/N on spectral lines in the Galactic center 4 times as fast as the current survey strategy.