On The Origin Of The Gamma Rays From The Galactic Center

TL;DR

This study reexamines gamma-ray emission from the Galactic Center using three years of Fermi-LAT data with Pass 7 ultraclean events to identify a spatially extended component peaking between $300MeV$ and $10GeV$. The authors find that the inner emission consists of a bright central source plus a spherically extended component, whose morphology is better described by a contracted dark matter profile with inner slope $\gamma\approx1.25$–$1.40$, and whose spectrum favors dark matter masses in the ranges $m_{DM}\approx7$–$12$ GeV (leptonic) or $m_{DM}\approx25$–$45$ GeV (hadronic) with $\langle\sigma v\rangle\sim3\times10^{-26}$ cm$^3$/s. They also assess alternative astrophysical origins, notably cosmic-ray protons accelerated by the SMBH, and find that while such protons could contribute, reproducing the rapid $200$–$700$ MeV rise is challenging; millisecond pulsars are unlikely to account for the extended excess given spectral and spatial constraints. The paper delivers conservative upper limits on the dark matter annihilation cross section that are competitive with or stronger than those from dwarf galaxies, and discusses implications for multi-wavelength signals (e.g., WMAP haze, ARCADE 2) and future tests to distinguish between DM and baryonic processes in the Galactic Center.

Abstract

The region surrounding the center of the Milky Way is both astrophysically rich and complex, and is predicted to contain very high densities of dark matter. Utilizing three years of data from the Fermi Gamma Ray Space Telescope (and the recently available Pass 7 ultraclean event class), we study the morphology and spectrum of the gamma ray emission from this region and find evidence of a spatially extended component which peaks at energies between 300 MeV and 10 GeV. We compare our results to those reported by other groups and find good agreement. The extended emission could potentially originate from either the annihilations of dark matter particles in the inner galaxy, or from the collisions of high energy protons that are accelerated by the Milky Way's supermassive black hole with gas. If interpreted as dark matter annihilation products, the emission spectrum favors dark matter particles with a mass in the range of 7-12 GeV (if annihilating dominantly to leptons) or 25-45 GeV (if annihilating dominantly to hadronic final states). The intensity of the emission corresponds to a dark matter annihilation cross section consistent with that required to generate the observed cosmological abundance in the early universe (sigma v ~ 3 x 10^-26 cm^3/s). We also present conservative limits on the dark matter annihilation cross section which are at least as stringent as those derived from other observations.

Figures (11)

• Figure 1: Contour maps of the gamma ray flux from the region surrounding the Galactic Center, as observed by the Fermi Gamma Ray Space Telescope. The left frames show the raw maps, while the center and right frames show the maps after subtracting known sources (not including the central source), and known sources plus emission from cosmic ray interactions with gas in the Galactic Disk, respectively. All maps have been smoothed over a scale of 0.5 degrees. See text for more details.
• Figure 2: The observed gamma ray flux (after subtracting known sources) averaged over $7^{\circ}<|l|<10^{\circ}$ as a function of galactic latitude (solid), compared to that predicted from the line-of-sight gas density (dashes). See text for details.
• Figure 3: The spectrum of the residual emission from the inner 5 degrees surrounding the Galactic Center, after subtracting the known sources and line-of-sight gas templates.
• Figure 4: Fits for the spectrum of the central emission, assuming a point-like source morphology, from the previous work of three different groups HG2Boyarsky:2010draharonian. Despite the different analysis approaches taken, these fits are all in reasonable agreement. The dashed line is the broken power-law fit to this spectrum as presented in Ref. aharonian.
• Figure 5: A comparison of the total residual emission found in this study (black) with the spectra of point-like emission (red) and extended emission (blue) (as in the case of annihilating dark matter with $\rho_{\rm DM}\propto r^{-1.34}$) as presented in Ref. Boyarsky:2010dr. This comparison supports our finding that this residual emission below $\sim 300$ MeV is consistent with a point-like source origin, while much of the emission at higher energies is indeed spatially extended.