Implications of the Fermi-LAT diffuse gamma-ray measurements on annihilating or decaying Dark Matter

TL;DR

This paper investigates how Fermi-LAT gamma-ray data constrains models of leptonically annihilating or decaying dark matter, showing that certain scenarios can explain multiple astrophysical observations without conflicting with existing gamma-ray measurements.

Contribution

It provides a comprehensive analysis of gamma-ray signals from dark matter annihilation or decay, considering various density profiles and substructure models, and assesses their compatibility with observational data.

Findings

Extragalactic gamma-ray contribution depends on background light models.

Decaying dark matter's Galactic signal always dominates over extragalactic.

Certain dark matter decay channels fit gamma-ray and cosmic-ray data simultaneously.

Abstract

We analyze the recently published Fermi-LAT diffuse gamma-ray measurements in the context of leptonically annihilating or decaying dark matter (DM) with the aim to explain simultaneously the isotropic diffuse gamma-ray and the PAMELA, Fermi and HESS (PFH) anomalous $e^\pm$ data. Five different DM annihilation/decay channels $2e$, $2\mu$, $2\tau$, $4e$, or $4\mu$ (the latter two via an intermediate light particle $\phi$) are generated with PYTHIA. We calculate both the Galactic and extragalactic prompt and inverse Compton (IC) contributions to the resulting gamma-ray spectra. To find the Galactic IC spectra we use the interstellar radiation field model from the latest release of GALPROP. For the extragalactic signal we show that the amplitude of the prompt gamma-emission is very sensitive to the assumed model for the extragalactic background light. For our Galaxy we use the Einasto, NFW and Isothermal DM density profiles and include the effects of DM substructure assuming a simple subhalo model. Our calculations show that for the annihilating DM the extragalactic gamma-ray signal can dominate only if rather extreme power-law concentration-mass relation $C(M)$ is used, while more realistic $C(M)$ relations make the extragalactic component comparable or subdominant to the Galactic signal. For the decaying DM the Galactic signal always exceeds the extragalactic one. In the case of annihilating DM the PFH favored parameters can be ruled out only if power-law $C(M)$ relation is assumed. For DM decaying into $2\mu$ or $4\mu$ the PFH favored DM parameters are not in conflict with the Fermi gamma-ray data. We find that, due to the (almost) featureless Galactic IC spectrum and the DM halo substructure, annihilating DM may give a good simultaneous fit to the isotropic diffuse gamma-ray and to the PFH $e^\pm$ data without being in clear conflict with the other Fermi-LAT gamma-ray measurements.