The preliminary design of the new space gamma-ray telescope GAMMA-400 for the energy range 100 MeV -3 TeV is presented. The angular resolution of the instrument, 1-2° at E γ ~100 MeV and ~0.01º at E γ > 100 GeV, its energy resolution ~1% at E γ > 100 GeV, and the proton rejection factor ~10 6 are optimized to address a broad range of science topics, such as search for signatures of dark matter, studies of Galactic and extragalactic gamma-ray sources, Galactic and extragalactic diffuse emission, gamma-ray bursts, as well as high-precision measurements of spectra of cosmic-ray electrons, positrons, and nuclei.

Our paper reviews the planned space-based gamma-ray telescope GAMMA-400 and evaluates in details its opportunities in the field of dark matter (DM) indirect searches. We estimated the GAMMA-400 mean sensitivity to the diphoton DM annihilation cross section in the Galactic center for DM particle masses in the range of 1–500 GeV. We obtained the sensitivity gain at least by 1.2–1.5 times (depending on DM particle mass) with respect to the expected constraints from 12 years of observations by Fermi-LAT for the case of Einasto DM density profile. The joint analysis of the data from both telescopes may yield the gain up to 1.8–2.3 times. Thus the sensitivity reaches the level of annihilation cross section ⟨ σ v ⟩γγ(mχ = 100 GeV) ≈ 10−28 cm3/s. This will allow us to test the hypothesized narrow lines predicted by specific DM models, particularly the recently proposed pseudo-Goldstone boson DM model. We also considered the decaying DM—in this case the joint analysis may yield the sensitivity gain up to 1.1–2.0 times reaching the level of DM lifetime τγν(mχ = 100 GeV) ≈ 2 ⋅ 1029 s. We estimated the GAMMA-400 sensitivity to axion-like particle (ALP) parameters by a potential observation of the supernova explosion in the Local Group. This is very sensitive probe of ALPs reaching the level of ALP-photon coupling constant gaγ ∼ 10−13 GeV−1 for ALP masses ma ≲ 1 neV. We also calculated the sensitivity to ALPs by constraining the modulations in the spectra of the Galactic gamma-ray pulsars due to possible ALP-photon conversion. GAMMA-400 is expected to be more sensitive than the CAST helioscope for ALP masses ma ≈ (1−−10) neV reaching gaγmin ≈ 2 ⋅ 10−11 GeV−1. Other potentially interesting targets and candidates are briefly considered too.

The GAMMA-400 gamma-ray telescope is designed to measure the fluxes of gamma rays and cosmic-ray electrons + positrons, which can be produced by annihilation or decay of the dark matter particles, as well as to survey the celestial sphere in order to study point and extended sources of gamma rays, measure energy spectra of Galactic and extragalactic diffuse gamma-ray emission, gamma-ray bursts, and gamma-ray emission from the Sun. The GAMMA-400 covers the energy range from 100 MeV to 3000 GeV. Its angular resolution is ~0.01º (E γ > 100 GeV), the energy resolution ~1% (E γ > 10 GeV), and the proton rejection factor ~10 6 . GAMMA-400 will be installed on the Russian space platform Navigator. The beginning of observations is planned for 2018.

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