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We describe a characteristic signature of dark matter (DM) annihilation or decay into gamma-rays. We show that if the total angular momentum of the initial DM particle(s) vanishes, and helicity suppression operates to prevent annihilation/decay into light fermion pairs, then the amplitude for the dominant 3-body final state f^+f^-gamma has a unique form dictated by gauge invariance. This amplitude and the corresponding energy spectra hold for annihilation of DM Majorana fermions or self-conjugate scalars, and for decay of DM scalars, thus encompassing a variety of possibilities. Within this scenario, we analyze Fermi LAT, PAMELA and HESS data, and predict a hint in future Fermi gamma-ray data that portends a striking signal at atmospheric Cherenkov telescopes (ACTs).
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The astrophysics community is considering plans for a variety of gamma-ray telescopes (including ACT and GRIPS) in the energy range 1--100 MeV, which can fill in the so-called MeV gap in current sensitivity. We investigate the utility of such detectors for the study of low-mass dark matter annihilation or decay. For annihilating (decaying) dark matter with a mass below about 140 MeV (280 MeV) and couplings to first generation quarks, the final states will be dominated by photons or neutral pions, producing striking signals in gamma-ray telescopes. We determine the sensitivity of future detectors to the kinematically allowed final states. In particular, we find that planned detectors can improve on current sensitivity to this class of models by up to a few orders of magnitude.
Recently reported tentative evidence for a gamma-ray line in the Fermi-LAT data is of great potential interest for identifying the nature of dark matter. We compare the implications for decaying and annihilating dark matter taking the constraints from continuum gamma-rays, antiproton flux and morphology of the excess into account. We find that higgsino and wino dark matter are excluded, also for nonthermal production. Generically, the continuum gamma-ray flux severely constrains annihilating dark matter. Consistency of decaying dark matter with the spatial distribution of the Fermi-LAT excess would require an enhancement of the dark matter density near the Galactic center.
We perform a new dark matter hot spot analysis using ten years of public IceCube data. In this analysis we assume dark matter self-annihilates to neutrino pairs and treat the production sites as discrete point sources. For neutrino telescopes these sites will appear as hot spots in the sky, possibly outshining other standard model neutrino sources. Comparing to galactic center analyses, we show that this approach is a powerful tool and capable of setting the highest neutrino detector limits for dark matter masses between 10 TeV and 100 PeV. This is due to the inclusion of spatial information in addition to the typically used energy deposition in the analysis.
Dwarf spheroidal galaxies are dark matter dominated systems, and as such, ideal for indirect dark matter searches. If dark matter decays into high-energy photons in the dwarf galaxies, they will be a good target for current and future generations of X-ray and gamma-ray telescopes. By adopting the latest estimates of density profiles of dwarf galaxies in the Milky Way, we revise the estimates dark matter decay rates in dwarf galaxies; our results are more robust, but weaker than previous estimates. Applying these results, we study the detectability of dark matter decays with X-ray and very-high-energy gamma-ray telescopes, such as eROSITA, XRISM, Athena, HAWC, and CTA. Our projection shows that all of these X-ray telescopes will be able to critically assess the claim of the 7 keV sterile neutrino decays from stacked galaxy clusters and nearby galaxies. For TeV decaying dark matter, we can constrain its lifetime to be longer than $sim$10$^{27}$-10$^{28}$ s. We also make projections for future dwarf galaxies that would be newly discovered with the Vera Rubin Observatory Legacy Survey of Space and Time, which will further improve the expected sensitivity to dark matter decays both in the keV and PeV mass ranges.
We point out the possibility to test the simplest scalar dark matter model at gamma-ray telescopes. We discuss the relevant constraints and show the predictions for direct detection, gamma line searches and LHC searches. Since the final state radiation processes are suppressed by small Yukawa couplings one could observe the gamma lines from dark matter annihilation.
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