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Constraints on light decaying dark matter candidates from 16 years of INTEGRAL/SPI observations

Francesca Calore, Ariane Dekker, Pasquale Dario Serpico, Thomas Siegert

TL;DR

This study addresses decaying light dark matter candidates in the MeV range by reanalyzing 16 years of INTEGRAL/SPI data with a dedicated DM spatial template. The authors implement two decay spectra, a monochromatic line and a final state radiation spectrum, within a joint Galactic DM template and astrophysical background fits, using 1D and 2D bound sets and global versus residual spectral analyses. They provide the strongest 95% CL bounds to date on the decay widths for both line and FSR channels and translate these into constraints on axion-like particle couplings and sterile neutrino mixing angles, highlighting the impact of spectral shape and analysis approach on the limits. The work underscores the MeV gap as a fertile ground for non-WIMP DM searches and advocates for future missions like COSI, ASTROGAM, and AMEGO to extend and solidify these constraints in the hard X-ray/soft gamma-ray band.

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.

Constraints on light decaying dark matter candidates from 16 years of INTEGRAL/SPI observations

TL;DR

This study addresses decaying light dark matter candidates in the MeV range by reanalyzing 16 years of INTEGRAL/SPI data with a dedicated DM spatial template. The authors implement two decay spectra, a monochromatic line and a final state radiation spectrum, within a joint Galactic DM template and astrophysical background fits, using 1D and 2D bound sets and global versus residual spectral analyses. They provide the strongest 95% CL bounds to date on the decay widths for both line and FSR channels and translate these into constraints on axion-like particle couplings and sterile neutrino mixing angles, highlighting the impact of spectral shape and analysis approach on the limits. The work underscores the MeV gap as a fertile ground for non-WIMP DM searches and advocates for future missions like COSI, ASTROGAM, and AMEGO to extend and solidify these constraints in the hard X-ray/soft gamma-ray band.

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 60 keV and 16 MeV experiencing two-body decays producing photon lines.
Paper Structure (7 sections, 9 equations, 5 figures)

This paper contains 7 sections, 9 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Methodology
  3. Results and implications
  4. Impact on specific models
  5. Axion-like particles
  6. Sterile neutrinos
  7. Discussion and conclusions

Figures (5)

  • Figure 1: Upper limits on the DM decay rate at 95% C.L. for the $2\gamma$ (left) and the FSR channel (right). Regions above the lines are excluded. In red (purple) we report the result of the total (NFW residuals only) spectral analysis by considering the 1D (2D) parameter space, shown as the solid (dashed) line.
  • Figure 2: Spectral data points (black) and the combination of fluxes leading to the $95\%$ upper limit: Unresolved astrophysical point sources are indicated in red histograms, the positronium emission in cyan histograms, the diffuse inverse Compton scattering in green histograms, the DM contribution in grey histograms. The sum of all best-fit components is displayed in purple. The following DM models are adopted: $2\gamma$ channel with $m_{\rm DM}=500$ keV (left panel), and FSR channel with $m_{\rm DM}=12$ MeV (right panel).
  • Figure 3: Upper limit on the ALP-to-photon coupling at 95% CL as a function of the ALP mass. Grey hatched regions represent previous X-ray and extragalactic background light constraints from Cadamuro:2011fdAxionLimits.
  • Figure 4: Upper limit on the ALP-to-electron coupling at 95% CL as a function of the ALP mass. Solid line: Direct bound from the FSR analysis. Dashed line: Expected indirect bounds from loop induced line contribution, valid at the order-of-magnitude level in absence of fine-tuned cancellations.
  • Figure 5: Upper limit on the mixing angle at 95% CL as a function of the sterile neutrino mass. Grey hatched region represents previous X-ray constraints from Boyarsky:2007geRoach:2022lgo.