Discovery of a new extragalactic source population of energetic particles

TL;DR

This paper reports a significant spectral hardening in the gamma-ray spectrum of Centaurus A, suggesting the presence of new high-energy particle sources such as dark matter spikes or millisecond pulsars, indicating novel gamma-ray production mechanisms.

Contribution

It introduces the first evidence that gamma-ray emission from active galaxies can be explained by mechanisms involving dark matter or pulsars, expanding understanding of high-energy astrophysical processes.

Findings

Spectral hardening observed at 2.6 GeV in Centaurus A

No overall flux variability detected over 8 years

Possible explanation involving dark matter spikes or millisecond pulsars

Abstract

We report the discovery of a statistically significant hardening in the Fermi-LAT $\gamma$-ray spectrum of Centaurus A's core, with the spectral index hardening from $\Gamma_{1}=2.73 \pm 0.02$ to $\Gamma_{1}=2.29 \pm 0.07$ at a break energy of ($2.6 \pm 0.3$) GeV. Using a likelihood analysis, we find no evidence for flux variability in Cen A's core lightcurve above or below the spectral break when considering the entire 8 year period. Interestingly, however, the first $\sim3.5$ years of the low energy lightcurve shows evidence of flux variability at the $\sim3.5 \sigma$ confidence level. To understand the origin of this spectral break, we assume that the low energy component below the break feature originates from leptons in Centaurus A's radio jet and we investigate the possibility that the high energy component above the spectral break is due to an additional source of very high energy particles near the core of Cen A. We show for the first time that the observed $\gamma$-ray spectrum of an Active Galactic Nucleus is compatible with either a very large localized enhancement (referred to as a spike) in the dark matter halo profile or a population of millisecond pulsars. Our work constitutes the first robust indication that new $\gamma$-ray production mechanisms can explain the emission from active galaxies and could provide tantalizing first evidence for the clustering of heavy dark matter particles around black holes.