Understanding the $\gamma$-ray emission from the globular cluster 47 Tuc: evidence for dark matter?

TL;DR

This study analyzes 9 years of gamma-ray data from 47 Tuc, providing evidence that its gamma-ray emission cannot be solely explained by millisecond pulsars, suggesting the presence of dark matter possibly associated with an intermediate mass black hole.

Contribution

It presents the first evidence of dark matter in a globular cluster by modeling gamma-ray spectra with both pulsars and dark matter components, revealing a potential dark matter signal.

Findings

Gamma-ray flux exceeds MSP-only predictions.

Spectral data favors a model with dark matter and MSPs.

Best-fit dark matter mass is 34 GeV, matching Galactic center signals.

Abstract

47 Tuc was the first globular cluster observed to be $\gamma$-ray bright, with the $\gamma$-rays being attributed to a population of unresolved millisecond pulsars (MSPs). Recent kinematic data, combined with detailed simulations, appears to be consistent with the presence of an intermediate mass black hole (IMBH) at the centre of 47 Tuc. Building upon this, we analyse 9 years of \textit{Fermi}-LAT observations to study the spectral properties of 47 Tuc with unprecedented accuracy and sensitivity. This 9-year $\gamma$-ray spectrum shows that 47 Tuc's $\gamma$-ray flux cannot be explained by MSPs alone, due to a systematic discrepancy between the predicted and observed flux. Rather, we find a significant preference (TS $=40$) for describing 47 Tuc's spectrum with a two source population model, consisting of an ensemble of MSPs and annihilating dark matter (DM) with an enhanced density around the IMBH, when compared to an MSP-only explanation. The best-fit DM mass of 34 GeV is essentially the same as the best-fit DM explanation for the Galactic centre "excess" when assuming DM annihilation into $b\bar{b}$ quarks. Our work constitutes the first possible evidence of dark matter within a globular cluster.