Two Emission Mechanisms in the Fermi Bubbles: A Possible Signal of Annihilating Dark Matter (1302.6589v1)

Abstract: We study the variation of the spectrum of the Fermi Bubbles with Galactic latitude. Far from the Galactic plane (|b| > 30 degrees), the observed gamma-ray emission is nearly invariant with latitude, and is consistent with arising from inverse Compton scattering of the interstellar radiation field by cosmic-ray electrons with an approximately power-law spectrum. The same electrons in the presence of microgauss-scale magnetic fields can also generate the the observed microwave "haze". At lower latitudes (b < 20 degrees), in contrast, the spectrum of the emission correlated with the Bubbles possesses a pronounced spectral feature peaking at 1-4 GeV (in E^2 dN/dE) which cannot be generated by any realistic spectrum of electrons. Instead, we conclude that a second (non-inverse-Compton) emission mechanism must be responsible for the bulk of the low-energy, low-latitude emission. This second component is spectrally similar to the excess GeV emission previously reported from the Galactic Center (GC), and also appears spatially consistent with a luminosity per volume falling approximately as r^-2.4, where r is the distance from the GC. We argue that the spectral feature visible in the low-latitude Bubbles is the extended counterpart of the GC excess, now detected out to at least 2-3 kpc from the GC. The spectrum and angular distribution of the signal is consistent with that predicted from ~10 GeV dark matter particles annihilating to leptons, or from ~50 GeV dark matter particles annihilating to quarks, following a distribution similar to the canonical Navarro-Frenk-White (NFW) profile. We also consider millisecond pulsars as a possible astrophysical explanation for the signal, as observed millisecond pulsars possess a spectral cutoff at approximately the required energy. Any such scenario would require a large population of unresolved millisecond pulsars extending at least 2-3 kpc from the GC.

• The paper identifies two emission mechanisms, with high-latitude gamma rays explained by inverse Compton scattering of cosmic-ray electrons.
• It reveals a distinct 1–4 GeV spectral feature at low latitudes that matches the morphology expected from dark matter annihilation.
• Alternative explanations like millisecond pulsars are considered, but spatial and spectral data strongly support a dark matter interpretation.

Analyzing Emission Mechanisms in the Fermi Bubbles

The paper "Two Emission Mechanisms in the Fermi Bubbles: A Possible Signal of Annihilating Dark Matter" by Dan Hooper and Tracy R. Slatyer presents a detailed paper of the gamma-ray emission from the Fermi Bubbles, lobes of high-energy particles extending from the Galactic Center. The research explores the spectral variation with Galactic latitude, distinguishing two distinct emission mechanisms and proposing the role of annihilating dark matter in contributing to the observed excess gamma-ray emissions.

Key Findings and Analysis

1. High-Latitude Emissions: The paper confirms that gamma-ray emissions far from the Galactic plane are consistent with inverse Compton scattering by cosmic-ray electrons with a power-law spectrum. These electrons, when interacting with microgauss-scale magnetic fields, account for both the gamma-ray emission and the observed microwave "haze." This supports a leptonic origin for the emissions in higher latitudinal regions of the Bubbles.
2. Low-Latitude Emissions: In contrast, at lower latitudes, a distinct spectral peak at 1-4 GeV is observed. The authors argue that this spectral feature cannot be produced by any plausible spectrum of cosmic-ray electrons via inverse Compton scattering, nor by hadronic processes involving cosmic-ray protons. This leads to the conclusion that a second, non-inverse-Compton mechanism is responsible for the bulk of the low-energy emissions in these regions.
3. Signal Interpretation: The paper suggests that the low-latitude emission component is spectrally and morphologically similar to the GeV excess previously reported in the Galactic Center. The distribution of this emission is found to be consistent with that which would result from 10 GeV dark matter particles annihilating into leptons or similarly, with 50 GeV particles annihilating into quarks. Specifically, the spatial distribution of this emission closely aligns with a profile slightly steeper than the canonical Navarro-Frenk-White (NFW) profile, exhibiting an inner slope around 1.2.
4. Alternative Explanations: While dark matter annihilation provides an attractive explanation, the authors also consider millisecond pulsars as a possible source. The paper notes that explaining this signal with pulsars would require a previously undetected population of unresolved millisecond pulsars extending over several kiloparsecs from the Galactic Center.

Implications and Future Directions

The implications of these findings are significant for both the understanding of Galactic cosmic-ray dynamics and the potential signals of dark matter annihilation. The research supports the presence of a new component of Galactic gamma-ray emission, aligning with models predicting dark matter phenomena. This adds weight to hypotheses that sub-GeV dark matter particles might have an observable influence on Galactic emissions, potentially guiding future observational and theoretical studies.

Moreover, this research prompts a re-evaluation of baryonic processes' influence on dark matter profiles, as indicated by the steepened inner slope found in simulations and empirically supported here. The investigation into alternative explanations such as pulsars also highlights areas where our understanding of galactic surveillances may need refinement.

Conclusion

This comprehensive paper of the Fermi Bubbles' emission mechanisms presents robust evidence for a non-standard source of gamma-ray emission at low Galactic latitudes, with parameters well aligned with annihilating dark matter scenarios. While millisecond pulsars remain a potential explanation, the research strongly suggests looking toward dark matter physics for future explorations. The presented work sets the stage for further investigations into galactic emissions and the indirect detection of dark matter, driving advancements in both astrophysical phenomena understanding and cosmic particle physics.