High-energy gamma-and cosmic-ray observations with future space-based GAMMA-400 gamma-ray telescope

. The future space-based GAMMA-400 gamma-ray telescope will be installed on the Navigator platform of the Russian Astrophysical Observatory. A highly elliptical orbit will provide observations for 7-10 years of many regions of the celestial sphere continuously for a long time ( ∼ 100 days). GAMMA-400 will measure gamma-ray ﬂuxes in the energy range from ∼ 20 MeV to several TeV and electron + positron ﬂuxes up to ∼ 20 TeV. GAMMA-400 will have an excellent separation of gamma rays from the background of cosmic rays and electrons + positrons from protons and an unprecedented angular ( ∼ 0 . 01 ◦ at E γ = 100 GeV) and energy ( ∼ 1% at E γ = 100 GeV) resolutions better than for Fermi-LAT, as well as ground-based facilities, by a factor of 5-10. Observations of GAMMA-400 will provide new fundamental data on discrete sources and spectra of gamma-ray emission and electrons + positrons, as well as the nature of dark matter.


Introduction
At present AGILE, Fermi-LAT, CALET, DAMPE perform observations of discrete gamma-ray sources in space.The third Fermi-LAT catalog (3FGL) contains 3033 sources for the energy range from 100 MeV to 300 GeV, but 33% of gamma-ray sources are unidentified [1].The groundbased facilities VERITAS, MAGIC, H.E.S.S., HAWC and others observe only 215 gamma-ray sources in the energy range above 100 GeV (http://tevcat.uchicago.edu/).It is important to note that the observational data were mainly obtained for the energy ranges < 100 GeV for Fermi-LAT and > 100 GeV for ground-based facilities and these data overlap poorly for many gamma-ray sources.The frontier range around 100 GeV is still very interesting for investigation.
Another very interesting and important goal in the studies of gamma-ray sky is indirect searches of dark matter (DM).WIMPs with mass between several GeV and several TeV are still considered as the most probable candidate.WIMPs can annihilate or decay with the production of gamma rays.This emission can have both continuous energy spectrum or mono-energetic narrow lines.Up to now, there are no data on DM gamma-ray lines from space-and ground-based instruments.

The GAMMA-400 physical scheme
The physical scheme of the GAMMA-400 gamma-ray telescope is shown in Fig. 1.
After interaction of incident gamma rays with the GAMMA-400 matter the backscattering omnidirectional particles (mainly 1-MeV photons and electrons) are arisen.Figure 2 shows the simulation result for primary 100-GeV gamma ray.In order to exclude backscattering particles, all scintillation detectors consist of two independent 10mm layers and fast timing methods are used.
The GAMMA-400 energy range for gamma-ray studies is from ∼ 20 MeV to several 1 TeV and up to ∼ 20 TeV for electrons + positrons.The field of view (FoV) for detecting particles from top is ±45 • .The geometrical factor for detecting electrons + positrons from vertical and four lateral directions is ∼3 m 2 sr.

The GAMMA-400 performance
Model calculations of the GAMMA-400 gamma-ray telescope performance were carried out using the "GEANT4.10.01.p02" software package.As a result of calculations, we obtained the following dependences: • the effective area vs the energy (Fig. 3), which is ∼ 5000 cm 2 for energies greater than 10 GeV; • the effective area vs the angle of incidence of particles for E γ = 1, 10, 100 GeV (Fig. 4); • the energy resolution vs the energy (Fig. 5).The energy resolution for E γ = 100 GeV is ∼ 1%; • the angular resolution vs the energy (Fig. 6).The angular resolution for E γ = 100 GeV is ∼ 0.01 • .
Using the combined information from all GAMMA-400 detector systems, it is possible to reach an effective rejection of protons from electrons.The methods to separate electron from protons presented in [6] are based on the difference of the development of hadronic and electromagnetic showers inside the instrument.For the current physical scheme the rejection factor for vertical protons is about 3×10 5 .

The GAMMA-400 astrophysical observatory
The GAMMA-400 astrophysical observatory will be installed onboard of the Navigator space platform, which is designed and manufactured by the Lavochkin Association and includes a gamma-ray telescope, an X-ray telescope and plasma detectors.
Using the Navigator space platform gives the GAMMA-400 experiment a highly unique opportunity for the near future gamma-and cosmic-ray science, since it allows us to install a scientific payload (mass of ∼ 4100 kg, power consumption of 2000 W, and telemetry downlink of 100 GB/day, with lifetime more than 7 years), which will provide GAMMA-400 by the means to significantly contribute as the next generation instrument for gamma-ray astronomy and cosmic-ray physics.
The GAMMA-400 experiment will be initially launched into a highly elliptical orbit (with an apogee of 300 000 km and a perigee of 500 km, with an inclination of 51.4 • ), with 7 days orbital period.Under the influence of gravitational disturbances of the Sun, Moon and the Earth after ∼ 6 months the orbit will transform to about an ap- proximately circular one with a radius of ∼ 200 000 km and will not suffer from the Earth's occultation and be outside the radiation belts.A great advantage of such an orbit is the fact that the full sky coverage will always be available for gamma-ray astronomy, since the Earth will not cover a significant fraction of the sky, as is usually the case for low-Earth orbit.Therefore, the GAMMA-400 source pointing strategy will hence be properly defined to maximize the physics outcome of the experiment.The launch of the GAMMA-400 space observatory is planned for the middle of the 2020s.

Comparison of GAMMA-400 with Fermi-LAT and ground-based facilities
The GAMMA-400 gamma-ray telescope has numerous advantages in comparison with the Fermi-LAT: • highly elliptical orbit (without the Earth's occultation and away from the radiation belts) allows us to continuously observe with an aperture of ±45 • different gamma-ray sources over a long period of time with the exposition greater by a factor of 8 than for Fermi-LAT operating in the sky-survey mode; • thanks to a smaller pitch (by a factor of 3) and analog readout in the coordinate silicon strip detectors, GAMMA-400 has an excellent angular resolution; • due to the deep (∼ 22 X 0 ) calorimeter, GAMMA-400 has an excellent energy resolution and can provide more reliably the detection of gamma rays up to several TeV for vertically incident events; • owing to the better gamma-ray separation from cosmic rays (in contrast to Fermi-LAT, the presence of a special trigger with event timing, time-of-flight system, twolayer scintillation detectors), GAMMA-400 is significantly well equipped to separate gamma rays from the background of cosmic rays and backscattering events.
GAMMA-400 will also have better angular and energy resolutions in the energy region 10-1000 GeV in comparison with current and future space-and ground-based

Figure 2 .
Figure 2. The interaction of the 100-GeV gamma ray with the GAMMA-400 matter with the formation of backscattering particles.Secondary gammas, positrons and electrons are marked by yellow, violet and blue colors respectively

Figure 3 .
Figure 3.The dependence of the effective area vs the energy for vertically incident particles

Figure 4 .Figure 5 .
Figure 4.The dependence of the effective area vs the angle of incidence of particles for E γ = 1 GeV; E γ = 10 GeV; E γ = 100 GeV