Data driven methods for the calibration of the energy scale of air Cherenkov telescopes

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The field of gamma-ray astronomy at the highest energies beyond 100 GeV has been driven by ground based imaging air Cherenkov telescopes (IACTs) with the discovery of numerous celestial particle accelerators, measurement of extra-galactic background light as well as a rich programme of fundamental physics including indirect searches for Dark Matter, axion-like particles, and Lorentz invariance violation. In order to fully exploit the potential of current instruments as well as for the planned Cherenkov telescope array (CTA), a careful calibration of the instruments is required. For CTA, the calibration and stability of the absolute energy calibration is required to be better than 10%. In the proposed project, we will develop further our previous work on cross-calibrating ground based with space based gamma-ray telescopes. Contrary to our previous work, the better energy coverage will allow to do a model-independent cross calibration using the overlap in energy. We suggest two new approaches to calibrate the energy scale with simulations. The method DC will use the direct Cherenkov light generated by cosmic-ray iron nuclei entering the atmosphere. The method PSF will exploit the excellent characterization of the point spread function with high statistics and its comparison with simulations: a shift of the energy scale in simulations and data leads to a mismatch of simulation and data. In turn, this can be used to tune the energy calibration. The approaches are complementary with each other and orthogonal to atmospheric monitoring approaches. The resulting systematic uncertainties are expected to be smaller than 10% on the absolute energy scale.