Astrophysical constraints from synchrotron emission on very massive decaying dark matter

TL;DR

This paper derives new constraints on very massive decaying dark matter by analyzing synchrotron emission from energetic electrons and positrons in the Galactic magnetic field, extending bounds up to the Planck scale.

Contribution

It introduces a novel method to constrain heavy decaying dark matter using synchrotron emission, considering various decay channels, magnetic field models, and density profiles.

Findings

Constraints are strongest for dark matter masses above 10^{12} GeV.

Synchrotron-based bounds complement gamma-ray and cosmic-ray limits.

Results depend on magnetic field configurations and decay channels.

Abstract

If the cosmological dark matter (DM) couples to Standard Model (SM) fields, it can decay promptly to SM states in a highly energetic hard process, which subsequently showers and hadronizes to give stable particles including $e^\pm$, $\gamma$, $p^{\pm}$ and $\nu\bar{\nu}$ at lower energy. If the DM particle is very heavy, the high-energy $e^\pm$, due to the Klein-Nishina cross section suppression, preferentially lose energy via synchrotron emission which, in turn, can be of unusually high energies. Here, we present previously unexplored bounds on heavy decaying DM up to the Planck scale, by studying the synchrotron emission from the $e^\pm$ produced in the ambient Galactic magnetic field. In particular, we explore the sensitivity of the resulting constraints on the DM decay width to (i) different SM decay channels, to (ii) the Galactic magnetic field configurations, and (iii) to various different DM density profiles proposed in the literature. We find that constraints from the synchrotron component complement and improve on constraints from very high-energy cosmic-ray and gamma-ray observatories targeting the prompt emission when the DM is sufficiently massive, most significantly for masses in excess of $10^{12}\text{ GeV}$.