Constraining DM properties with SPI

TL;DR

This work leverages the high-resolution SPI spectrometer on INTEGRAL to search for radiative decays of decaying dark matter in the Milky Way halo, focusing on sterile neutrino DM across $M_{\rm DM} \in [40\ \mathrm{keV}, 14\ \mathrm{MeV}]$. By modeling DM halo profiles (NFW and isothermal), computing line-of-sight DM column densities, and employing a background-subtracted line search (ON–OFF method) that accounts for instrumental variations, the authors derive 3$\sigma$ upper limits on the DM decay line flux and translate these into constraints on the mixing angle $\sin^2 2\theta$ and DM lifetimes. No DM decay line is detected; the resulting bounds tighten existing limits in the 40 keV–7 MeV range and highlight the need for improved background subtraction and imaging to access weaker lines. The findings provide robust, model-dependent constraints on radiatively decaying DM and outline pathways to enhance sensitivity for future hard X-ray/soft gamma-ray line searches.

Abstract

Using the high-resolution spectrometer SPI on board the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), we search for a spectral line produced by a dark matter(DM) particle with a mass in the range 40keV < M_DM < 14MeV, decaying in the DM halo of the Milky Way. To distinguish the DM decay line from numerous instrumental lines found in the SPI background spectrum, we study the dependence of the intensity of the line signal on the offset of the SPI pointing from the direction toward the Galactic Centre. After a critical analysis of the uncertainties of the DM density profile in the inner Galaxy, we find that the intensity of the DM decay line should decrease by at least a factor of 3 when the offset from the Galactic Centre increases from 0 to 180 degrees. We find that such a pronounced variation of the line flux across the sky is not observed for any line, detected with a significance higher than 3 sigma in the SPI background spectrum. Possible DM decay origin is not ruled out only for the unidentified spectral lines, having low (~3 sigma) significance or coinciding in position with the instrumental ones. In the energy interval from 20 keV to 7 MeV, we derive restrictions on the DM decay line flux, implied by the (non-)detection of the DM decay line. For a particular DM candidate, the sterile neutrino of mass MDM, we derive a bound on the mixing angle.

Figures (13)

• Figure 1: Comparison of sensitivity towards the search of the narrow DM decay line for different instruments with the wide FoV. Diagonal straight lines show the improvement of sensitivity (by a factor, marked on the line) as compared with the HEAO-I A4 low energy detector (LED), taken as a reference.
• Figure 2: The geometry of the SPI FoV.
• Figure 3: The effective area of the SPI detector for an on-axis source, as a function of the photon energy. The plot is produced by collective the on-axis effective areas of the 17 SPI detectors from the instrumental characteristics files.
• Figure 4: Dependence of the effective area on the off-axis position of a (point) source.
• Figure 5: Expected column density for various DM profiles: favored NFW profile (red thick solid line); NFW profile with the maximal disk (model $A_2$, see Table \ref{['tab:nfw']}) -- blue solid line; cored (isothermal) profile -- green thick dashed line; constant density within $r_\odot$ -- black dashed line).