Two new articles appeared this week on arXiv have deepened our understanding of the mysterious gamma-ray excess glowing at the centre of our Galaxy.
The masked Galactic plane and the hunt for dark matter
The first study [1] delves into the intricacies of the GCE’s spatial distribution, emphasising the role of models for the Galactic diffuse emission, particularly in masking the Galactic plane and point sources. One prevailing hypothesis suggests that the GCE may be a result of the annihilation of dark matter (DM) particles, leading to a spherically symmetric morphology. However, the researchers argue that recent claims favouring a DM-motivated template rely heavily on a specific Galactic bulge template, which is outperformed by alternative templates.
To test the validity of these claims, the scientists introduce a non-parametric model of the Galactic bulge derived from the VVV survey. Surprisingly, this alternative model provides a significantly better fit for the GCE than DM-motivated templates, independent of the chosen model for describing the diffuse Galactic emission. Even when additional freedom is allowed in the background models, favouring non-parametric modulation, the preference for a boxy bulge morphology remains robust.
A new perspective: Bulge morphology and sub-threshold point sources
The second study [2] employs an innovative approach, combining skyFACT adaptive template fitting with 1pPDF pixel-count statistics, to analyse Fermi-LAT data at energies above 10 GeV. The findings show a strong preference for a bulge morphological model over the dark matter hypothesis, even when unresolved point sources are included in the fit, and only considering high-energy data. Moreover, no evidence for an additional dark matter template is found atop the bulge component.
Through the 1pPDF analysis, the researchers reconstruct a consistent population of faint point sources down to at least 10-12 ph/cm2s. Interestingly, between 10-12 – 10–11 ph/cm2s, the measured number of point sources exceeds those documented in the Fermi 4FGL catalog. This leads to the intriguing possibility that a population of point sources, potentially millisecond pulsars, might play a role in explaining the high-energy tail of the GCE.
A consistent narrative emerges
Both studies contribute valuable insights to the ongoing quest to decipher the origin of the Galactic Center Excess. The non-parametric model of the Galactic bulge and the preference for a bulge morphological model in high-energy data challenge prevailing dark matter hypotheses. Instead, the results suggest the involvement of faint point sources, possibly millisecond pulsars, as contributors to the enigmatic gamma-ray signal. As technology advances and our understanding of the universe deepens, the mysteries surrounding the Galactic Center Excess may soon unravel, ushering in a new era of astrophysical discovery.
Related links
[1] Song D., et al., Robust inference of the Galactic centre gamma-ray excess spatial properties, https://arxiv.org/abs/2402.05449
[2] Manconi S., Calore F. and Donato F., The Galactic center excess at the highest energies: morphology and photon-count statistics, https://arxiv.org/abs/2402.04733
Image credit: NASA Goddard/A. Mellinger (Central Michigan Univ.) and T. Linden (Univ. of Chicago)
Francesca Calore 