Synchrotron Self-Compton Process for Constraining sub-GeV Dark Matter in Omega Centauri via SKA

TL;DR

This work targets sub-GeV dark matter by exploiting the synchrotron self-Compton (SSC) channel, where MeV-scale DM annihilation produces $e^+e^-$ that first emit synchrotron radiation and then upscatter those photons via inverse Compton processes. Focusing on Omega Centauri and the sensitivity of the Square Kilometre Array (SKA), the authors solve a diffusion–loss framework for the injected electrons, compute the SSC and synchrotron spectra, and compare with SKA reach to derive competitive upper limits on the annihilation cross section, $⟨σv⟩$, down to about $10^{-30}$ cm$^3$ s$^{-1}$ in the tens-of-MeV range, with optimistic parameter choices potentially reaching $10^{-32}$ cm$^3$ s$^{-1}$. The analysis includes uncertainties from DM density profiles, magnetic field, and diffusion, finding that diffusion dominates the error budget while the density profile has a smaller impact. Overall, SSC emission in the MeV gap offers a robust and promising indirect detection channel, potentially surpassing existing limits and complementing other probes in the MeV mass regime.

Abstract

The search for the particle identity of dark matter (DM) continues to be a primary objective in modern physics. In this field, the sub-GeV mass range of DM detection remains a crucial yet challenging window. We investigate synchrotron self-Compton (SSC) emission from electrons and positrons produced by MeV-scale DM annihilation as a novel indirect detection channel. Focusing on the globular cluster Omega Centauri and the sensitivity of the Square Kilometre Array, we derive constraints on the annihilation cross section reaching $\langleσv\rangle \sim 10^{-30}\,\rm{cm}^{3}\,\rm{s}^{-1}$ in the tens-of-MeV range. Furthermore, constraints could even reach below $\langleσv\rangle \sim 10^{-32}\,\rm{cm}^{3}\,\rm{s}^{-1}$ for extreme parameter choices. Remarkably, even under deliberately conservative astrophysical assumptions, this channel outperforms existing indirect limits, establishing SSC emission as a robust probe of sub-GeV DM.

Figures (3)

• Figure 1: The energy flux of SSC (black line) compared with the energy flux of synchrotron (blue line). Here we choose $m_\chi = 50~\rm{MeV}$, $D_0 = 10^{30}~\rm {cm^{2}~s^{-1}}$, $\langle\sigma v\rangle=10^{-30} ~\rm{cm^{3}~s^{-1}}$, $B=1~\rm {\mu G}$. The green and cyan regions represent the sensitivity of SKA phase1 and phase2 with 100 hours of observation, respectively Braun:2019gdoWang:2023sxr.
• Figure 2: Our constrains of DM annihilation cross-section with $D_0 = 10^{30}~\rm {cm^{2}~s^{-1}}$ and $B=1~\rm {\mu G}$ (black solid line), compared with limits from CMB data Slatyer:2015jlaLeane:2018kjk (red dashed line); Voyager cosmic-ray data Boudaud:2016mos (blue solid line); AMS-02 cosmic-ray data Wang:2025jhy (purple solid line); secondary X-ray emissions Cirelli:2020bpcCirelli:2023tnx (green dotted line) and synchrotron radiation Wang:2023sxr (yellow solid line).
• Figure 3: Our fiducial constrains of DM annihilation cross-section (black solid line), together with the effects of astrophysical uncertainties. The green, blue, and red bands show variations due to the DM density profile (Profiles 2, 3), diffusion coefficient ($D_0=10^{26}$--$10^{30}\,\rm{cm}^{2}\,\rm{s}^{-1}$), and magnetic field ($1$--$10\,\mu\rm{G}$), respectively.