Abstract
Context. Measurement of diffuse γ-ray emission from the Milky Way with Imaging Atmospheric Cherenkov Telescopes (IACT) is difficult because of the high level of charged cosmic ray background and the small field of view. Aims. We show that such a measurement is nevertheless possible in the energy band 10−100 TeV. Methods. The minimal charged particle background for IACTs is achieved by selecting the events to be used for the analyses of the cosmic ray electrons. Tight cuts on the event quality in these event selections allow us to obtain a sufficiently low background level to allow measurement of the diffuse Galactic γ-ray flux above 10 TeV. We calculated the sensitivities of different types of IACT arrays for the Galactic diffuse emission measurements and compared them with the diffuse γ-ray flux from different parts of the sky measured by the Fermi Large Area Telescope below 3 TeV and with the astrophysical neutrino signal measured by IceCube telescope. Results. We show that deep exposure of existing IACT systems is sufficient for detection of the diffuse flux from all the Galactic Plane up to Galactic latitude |b| ∼ 5°. The Medium Size Telescope array of the CTA will be able to detect the diffuse flux up 30° Galactic latitude. Its sensitivity will be sufficient for detection of the γ-ray counterpart of the Galactic component of the IceCube astrophysical neutrino signal above 10 TeV. We also propose that a dedicated IACT system composed of small but wide-field-of-view telescopes could be used to map the 10−100 TeV diffuse γ-ray emission from across the whole sky. Conclusions. Detection and detailed study of diffuse Galactic γ-ray emission in the previously unexplored 10−100 TeV energy range is possible with the IACT technique. This is important for identification of the Galactic component of the astrophysical neutrino signal and for understanding the propagation of cosmic rays in the interstellar medium.
Highlights
The Milky Way galaxy is the strongest γ-ray source on the sky
We calculated the sensitivities of different types of Imaging Atmospheric Cherenkov Telescopes (IACT) arrays for the Galactic diffuse emission measurements and compared them with the diffuse γ-ray flux from different parts of the sky measured by the Fermi Large Area Telescope below 3 TeV and with the astrophysical neutrino signal measured by IceCube telescope
We show that the IACT technique could be used measure the diffuse Galactic γ-ray flux in the energy range 10−100 TeV overlapping with the range of the IceCube measurements of the astrophysical neutrino flux
Summary
The Milky Way galaxy is the strongest γ-ray source on the sky. Its flux is dominated by the diffuse emission produced by interactions of cosmic ray atomic nuclei and electrons all across the interstellar medium. Its spectrum shows a puzzling behaviour extending up to the highest energies as a power law with the slope dN/dE ∝ E−Γγ , Γγ 2.4, which is harder than the slope of the locally measured cosmic ray spectrum (2.6 < ΓCR < 2.9) (Tanabashi et al 2018) This is surprising because the diffuse emission flux in the TeV energy range is expected to be dominated by the pion decay emission from interactions of cosmic ray nuclei. Model-independent identification of the Galactic diffuse γ-ray+neutrino emission signal would help to clarify the peculiarities of the cosmic ray propagation in the multi-PeV energy range around the knee of the cosmic ray spectrum Such identification is not possible with the IceCube data alone because of the low statistics of the signal: only several tens of neutrinos are detected by IceCube in the energy range above 30 TeV. We argue that the IACT technique could be optimised for the measurement of diffuse γ-ray emission and show that a system of small- but wide-FoV IACTs would be able to measure the diffuse γ-ray flux from both lowand high-Galactic-latitude regions on the sky in an energy range overlapping with that of the astrophysical neutrino signal
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