[Paper Review] Constraints on light decaying dark matter candidates from 16 years of INTEGRAL/SPI observations

Francesca Calore, Ariane Dekker+2 more|ArXiv.org|Sep 13, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This study uses 16 years of INTEGRAL/SPI data with a dark matter spatial template to set the strongest constraints to date on light decaying dark matter candidates—specifically sterile neutrinos and axion-like particles—between 60 keV and 16 MeV, achieving 95% confidence level upper limits on their decay rates, particularly for monochromatic photon lines and continuum radiation from two-body decays.

ABSTRACT

We apply the recently developed analysis of 16 years of INTEGRAL/SPI data including a dark matter spatial template to derive bounds on dark matter candidates lighter than WIMPs (like sterile neutrinos or axion-like particles) decaying into line or continuum electromagnetic final state channels. The bounds obtained are the strongest to date for dark matter masses between $\sim $60 keV and $\sim$16 MeV experiencing two-body decays producing photon lines.

Motivation & Objective

• To derive robust upper limits on decaying dark matter candidates lighter than WIMPs, focusing on sterile neutrinos and axion-like particles.
• To improve constraints in the MeV energy range, where indirect detection via gamma-ray lines and continuum emission is theoretically plausible but observationally challenging.
• To assess the impact of spectral shape (line vs. broad continuum) on derived bounds in a multi-component spectral fitting framework.
• To evaluate the reliability of simplified residual-based analyses versus full global spectral fits in deriving dark matter constraints.

Proposed method

• Performs a maximum likelihood analysis using the 3ML package to fit spatial and spectral templates simultaneously to 16 years of SPI data.
• Includes a dark matter spatial template based on the NFW profile, treating it as an independent component alongside known astrophysical backgrounds (e.g., 511 keV positronium line, inverse Compton scattering).
• Convolved model fluxes with SPI’s energy redistribution matrix to account for instrumental response functions.
• Analyzes two decay channels: monochromatic photon lines (e.g., sterile neutrino decay to νγ) and continuum emission from two-body decays (e.g., νs → νa e+e−).
• Applies a global fit across all energy bins (30 keV–8 MeV), allowing for degeneracy between DM and astrophysical components.
• Derives 95% confidence level upper limits on decay rates by comparing model predictions to observed data, with no significant DM-like excess detected.

Experimental results

Research questions

• RQ1 What are the strongest constraints on decaying dark matter candidates with masses between 60 keV and 16 MeV using 16 years of INTEGRAL/SPI data?
• RQ2 How do the derived bounds depend on the spectral shape of the dark matter decay—specifically, how do line-like and continuum-like spectra affect the sensitivity?
• RQ3 To what extent do simplified residual-based analyses overestimate or underestimate bounds compared to a full global spectral fit?
• RQ4 How do the constraints on sterile neutrinos and axion-like particles compare to previous limits, particularly in the MeV mass range?
• RQ5 What is the impact of astrophysical background degeneracy on the reliability of dark matter bounds in multi-component fitting?

Key findings

• The study sets the strongest constraints to date on decaying dark matter candidates in the 60 keV to 16 MeV mass range, particularly for two-body decays producing photon lines.
• For sterile neutrinos, the bounds on the decay rate are strongest around 100 keV to 1 MeV, with upper limits on the branching ratio into a photon and active neutrino that are competitive with or exceed previous limits.
• The analysis reveals that simplified residual-based methods can overestimate or underestimate bounds by up to a factor of ~6 compared to a full global fit, depending on spectral degeneracy with astrophysical components.
• The constraints are most stringent for monochromatic lines, with the upper limit on the decay rate for a 100 keV sterile neutrino reaching ~10^-32 s^-1.
• For axion-like particles, the bounds are strongest in the 100 keV–1 MeV range, where the spectral shape is most distinguishable from background components.
• The results highlight the importance of using full-spectrum fitting over simplified residual analysis to avoid biased or overly optimistic constraints.