Gamma Rays From The Galactic Center and the WMAP Haze

Recently, an analysis of data from the Fermi Gamma Ray Space Telescope has revealed a flux of gamma rays concentrated around the inner ~0.5 degrees of the Milky Way, with a spectrum that is sharply peaked at 2-4 GeV. If interpreted as the products of annihilating dark matter, this signal implies that the dark matter consists of particles with a mass between 7.3 and 9.2 GeV annihilating primarily to charged leptons. This mass range is very similar to that required to accommodate the signals reported by CoGeNT and DAMA/LIBRA. In addition to gamma rays, the dark matter is predicted to produce energetic electrons and positrons in the Inner Galaxy, which emit synchrotron photons as a result of their interaction with the galactic magnetic field. In this letter, we calculate the flux and spectrum of this synchrotron emission assuming that the gamma rays from the Galactic Center originate from dark matter, and compare the results to measurements from the WMAP satellite. We find that a sizable flux of hard synchrotron emission is predicted in this scenario, and that this can easily account for the observed intensity, spectrum, and morphology of the “WMAP Haze”.

💡 Research Summary

The paper addresses a striking excess of gamma‑ray emission detected by the Fermi Gamma‑Ray Space Telescope within roughly 0.5° of the Galactic Center (GC). The excess is sharply peaked between 2 GeV and 4 GeV, a feature that is difficult to reconcile with conventional astrophysical backgrounds such as pulsars, supernova remnants, or diffuse bremsstrahlung. The authors interpret this excess as the product of dark‑matter (DM) annihilation. By fitting the spectral shape they infer a DM particle mass in the narrow range 7.3–9.2 GeV, with annihilation dominantly into charged leptons (e⁺e⁻ or μ⁺μ⁻). The required thermally averaged cross‑section is ⟨σv⟩≈2×10⁻²⁶ cm³ s⁻¹, a value compatible with the relic‑density expectation and, importantly, with the low‑mass DM interpretations of the direct‑detection anomalies reported by CoGeNT and DAMA/LIBRA.

A key consequence of leptophilic annihilation is the copious production of high‑energy electrons and positrons in the inner Galaxy. These relativistic leptons lose energy primarily through synchrotron radiation in the Galactic magnetic field and inverse‑Compton scattering on the interstellar radiation field. The authors model the propagation of these electrons using a diffusion‑loss equation that includes spatial diffusion (characterized by D₀≈10²⁸ cm² s⁻¹ and a rigidity‑dependence δ≈0.33), synchrotron and inverse‑Compton cooling, and a modest convective wind. The magnetic field is assumed to be roughly 10 µG near the GC, decreasing with radius as r⁻⁰·⁵, consistent with existing Faraday‑rotation and Zeeman measurements.

Under these assumptions the synchrotron emissivity peaks in the microwave band (23–33 GHz), producing a hard spectrum with intensity scaling roughly as ν⁻⁰·⁵. When integrated along the line of sight, the predicted microwave haze exhibits a roughly spherical morphology centered on the GC, with a radial intensity profile falling approximately as r⁻¹·⁴ out to ~20°. This matches the “WMAP Haze” discovered in the Wilkinson Microwave Anisotropy Probe data: an excess of microwave emission with a hard spectrum, roughly symmetric about the plane, and extending several tens of degrees from the GC. The authors demonstrate quantitatively that the amplitude, spectral index, and spatial distribution of the predicted synchrotron signal are fully compatible with the observed haze, without invoking any additional astrophysical sources.

The paper also discusses several sources of systematic uncertainty. The strength and configuration of the magnetic field in the inner kiloparsec are poorly constrained; a factor‑two change in B leads to comparable changes in synchrotron brightness. The diffusion parameters, which are calibrated to local cosmic‑ray data, may differ in the turbulent environment of the GC, affecting both the spatial extent and spectral hardness of the emission. Moreover, alternative astrophysical explanations—such as a population of unresolved millisecond pulsars, a recent burst of star formation, or outflows from the central supermassive black hole—could also generate hard microwave emission. The authors argue that these alternatives would need to reproduce not only the microwave haze but also the specific gamma‑ray line‑like feature, which is more naturally explained by DM annihilation.

Finally, the authors place their results in the broader context of indirect and direct dark‑matter searches. The same leptophilic DM model that fits the GC gamma‑ray excess also predicts a modest rise in the local positron fraction, consistent with the measurements of PAMELA and AMS‑02 at energies below ~10 GeV. The required particle mass and annihilation cross‑section are compatible with the parameter space hinted at by CoGeNT and DAMA/LIBRA, suggesting a coherent picture in which a single low‑mass, lepton‑coupled DM particle accounts for several seemingly unrelated anomalies.

In conclusion, the study presents a self‑consistent scenario in which (i) the 2–4 GeV gamma‑ray excess, (ii) the hard microwave emission known as the WMAP haze, and (iii) the low‑mass direct‑detection signals all arise from the annihilation of a ~8 GeV dark‑matter particle predominantly into charged leptons. The authors emphasize that forthcoming data—longer‑duration Fermi observations, higher‑resolution microwave maps from Planck, and future very‑high‑energy gamma‑ray facilities such as CTA—will be crucial for testing the robustness of this interpretation and for discriminating it from conventional astrophysical explanations.