In the last decade PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, located at the Resurs-DK1 Russian satellite), AMS-02 (Alpha Magnetic Spectrometer on the International Space Station) and the Large Area Telescope (LAT) on board of the Gamma-Ray Space Telescope (Fermi) have observed the Universe through the study of the Galactic cosmic rays and gamma rays. In particular, the Fermi telescopes have provided detailed maps of the gamma-ray sky covering an unprecedented energy range from about 20 MeV up to more than 500 GeV and spanning a very large portion of the sky (around 20%)(see Fig. 1).
Figure 1: Gamma-ray sky provided by the Fermi telescope. Image credit:Nasa.
The high energy γ-ray sky provides a unique window on a variety of fundamental topics in cosmology, astrophysics and particle physics, and it is crucial to constrain theoretical models of possible contributions from astrophysical or even exotic origins. The γ-ray sky is given by the superposition of γ-ray emission from point-sources (detected and undetected, as for example active galactic nuclei, blazars, starburst galaxies, pulsars and γ-ray bursts) and diffuse emission processes. In particular, the γ-ray emission from the galaxy is the brightest source of photons along the galactic disk. The emission is due to pion decay, bremsstrahlung radiation by cosmic ray electrons, and inverse Compton scattering between relativistic electrons and soft photons.
Above the standard astrophysical background and the already known gamma-ray sources, Fermi-LAT data revealed a mysterious excess. Several groups have independently analyzed the Fermi sky and found the excess both in the very inner few degrees of the Galaxy (Macias and Gordon, arXiv:1312.6671, Abazajian et al., 1402.4090) as well as in a region that extends up to 10 degrees (Daylan et al., arXiv:1402.6703). The spectral and morphological properties have been derived, and both seem to be consistent with a signal whose spectrum peaks around few GeV and with a luminosity per volume following a steep power-law and compatible with being spherically symmetric around the Galactic Center.
Several astrophysical processes have been proposed in order to explain the signal: emission from an unresolved population of sources, like millisecond pulsars, interactions between gas and protons accelerated by a super massive black hole sitting at the galactic centre, burst-like events during an active past of our Galaxy.
However a tantalizing hypothesis arises from cosmology: dark matter, which is one of the building blocks of the cosmological standard model and represents the 85% of the matter content of the Universe, could explain this excess. Indeed, Weakly Interacting Massive Particles, the most popular dark matter candidates, may annihilate and contribute to the gamma-ray flux trough their annihilation products. This signal could be seen as an excess peaking at a few GeV (in the case of a light DM particle with mass in the range 30 - 60 GeV) and extending to higher energies, up to an end-point which depends on the precise WIMP mass.
In order to robustly confirm the existence of this Galactic center excess and and firmly characterize its properties and uncertainties, the authors perform a meticulous subtraction of the main background (the aforementioned diffuse galactic emission) and, therefore, an accurate extraction of the anomalous signal in the region called inner Galaxy corresponding to galactic latitudes 2º≤ |b| ≤ 20º and to galactic longitudes |l| < 20º. Due to the lack of knowledge of the physical conditions in that particular region of the Milky Way, a large number of systematic uncertainties are involved in modelling the background. In this paper the systematic effects are taken into account, in order to ensure a correct characterization of the excess.
The major results of the paper are summarized here:
1) The authors confirm the existence of an extended emission in the inner part of the Galaxy. They found that the spectral and morphological properties of the emission are not sensitive to extreme variations of the galactic diffuse emission.
2) By fully including the systematic uncertainties in the analysis, the authors demonstrate that the excess emission is compatible with a spherically symmetric and spectrally uniform signal, that extends at least up to 10 degrees in latitude. The extension to high latitudes is a very important ingredient for discriminating among different interpretations of the signal.
3) The results of the spectral fits to the derived excess show that a broken power-law is well compatible with the data as well as a signal from DM annihilation.
The broken power-law is a standard spectrum for diffuse astrophysical emission processes and might be, for example, explained by the recently suggested interpretation in terms of a leptonic-burst event (Petrovic et al., arXiv:1405.7928).
The emission from an unresolved population of millisecond pulsars is currently disfavored by the data because of the fact that they would contribute at most to 5-10% of the emission. Moreover, the extension of the signal up to at most 10 degrees makes more difficult to explain it in terms of millisecond pulsars.
A signal from dark matter annihilation?
The statistical analysis shows a good agreement between dark matter annihilation and the Galactic centre excess, even if this is not strongly evident for the Fig.2. The reason is due to the details of the spectral fit analysis, that takes into account the correlation of systematic uncertainties. The authors consider different annihilation channels: in quarks (bottom and charm) and heavy leptons (tau). In particular, for the bb annihilation channel, they obtain a dark matter mass in the range 43.6-55.4 GeV and a velocity-averaged annihilation cross-section of <σ v >=1.76 ± 0.28 x 10-26 cm3 s-1. As a last remark, we would stress that for the compatibility between signal and hypothesis not only the spectrum is important but also all the morphology, namely the spherical symmetry and the extension to high latitudes.
To sum up, the GeV gamma-ray excess has caused a large excitement in the scientific community. Further developments of the analysis and a better handling of the astrophysical uncertainties are expected to shed light on the origin of this (still unknown) phenomenon.
Figure 2: Spectrum of the GCE emission, together with statistical and systematical er-
rors.The black dots with the error bars show the energy spectrum of the excess revealed by
Fermi in the galactic center. The lines show different plausible fitting models. The red dotted
line is a power law with an exponential cut-off, while the pink dashed line is a broken power
law. The green and the blue dash-dot lines are the best-fit models of two different dark matter
annihilation channels (either bottom quark or tau lepton).
Text by Maria Archidiacono and Ninetta Saviano