Mapping large-scale diffuse gamma-ray emission in 10-100 TeV band with Cherenkov telescopes

Abstract

Measurement of diffuse gamma-ray emission from the Milky Way with Imaging Atmospheric Cherenkov Telescopes (IACT) is difficult because of high level of charged cosmic ray background and small field-of-view. We show that such a measurement is nevertheless possible in the energy band 10-100 TeV. The minimal charged particle background for IACTs is achieved in event selections used for the analyses of the cosmic ray electrons. Tight cuts on the event quality in these event selections allow to achieve the background level sufficiently low to allow the measurements of the diffuse Galactic gamma-ray flux above 10 TeV. We calculate the sensitivities of different types of IACT arrays for the Galactic diffuse emission measurements and compare them with the diffuse gamma-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 deep exposure of existing IACT systems is sufficient for detection of the diffuse flux from all the Galactic Plane up to Galactic latitude |b|~ degrees. Medium Size Telescope array of CTA will be able to detect the diffuse flux up 30 degree Galactic latitude. Its sensitivity will be sufficient for detection of the gamma-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 for mapping the 10-100 TeV the diffuse gamma-ray emission from across all the sky.