The potential of Imaging Atmospheric Cherenkov Telescope arrays for morphological and spectrometric studies

In recent years, the current generation of ground-based gamma-ray telescopes has proved the great potential of the stereoscopic arrays of Imaging Atmospheric Cherenkov Telescopes (IACTs) as effective multifunctional tools for spectral and morphological studies of gamma-ray sources at energies from a few tens of GeV up to tens of TeV. The next generation IACT observatory, namely the Cherenkov Telescope Array (CTA), is designed to pursue the next step of development in terms of instrument performance. Consequently, a strong impact on our understanding of the non-thermal Universe is foreseen. In this work, the actual potential of the next generation IACT arrays in general, and CTA, in particular, to perform morphological and spectrometric studies of very high energy (VHE) gamma-ray sources is explored. In addition to the ideal (Gaussian form) point spread function (PSF), the impact of more realistic non-Gaussian PSFs with extended tails has been considered. Simulations of isolated point-like and extended sources have been performed, motivated by the fact that morphological studies of complex regions of extended emission or regions crowded by gamma-ray sources, such as the region of the Galactic Center, represent one of the main challenges to be faced by the the upcoming observatory. The simulations have been used to understand the response of the instrument under different assumptions regarding the PSF model. When the contribution of the tails accounts for ≥ 5% of the Gaussian PSF, the reconstruction of the source morphology is significantly compromised, due to the artificial emission induced by the presence of the tails. Then, the capability of the instrument to resolve multiple sources has been analyzed and the corresponding instrument sensitivities calculated. It has been shown that, depending on the PSF shape and on the distance to the test source, the presence of a nearby gamma-ray emitter increases the minimum detectable flux up to an order of magnitude with respect to what foreseen by CTA in case of an isolated point-like source and Gaussian PSF. The CTA expectations on the flux sensitivity are found to be reduced also in correspondence to isolated extended objects. The worsening is especially significant in the low energy domain: around few tens of GeV, a degradation of the sensitivity by an order of magnitude or more is expected. These results should be taken into account when planning the observations with future IACT arrays, especially in the densely populated part of the Galactic plane, where the chance of clustering of two or more gamma-ray sources within 1 deg is high and many of the objects are found to be better described as extended sources, rather than point-like. Finally, the potential of CTA for spectrometric studies of VHE gamma-ray objects has been evaluated. Besides the featureless, e.g. pure power-law, energy spectra, the cases of spectra with exponential cutoffs and broken power-law energy distributions have been examined. It has been demonstrated that the significant improvement expected for CTA regarding both the detection area and the energy resolution will allow the reconstruction of energy spectra with sharp spectral features with great accuracy. Sharp cutoffs or breaks around tens of TeV could be properly reconstructed, provided adequate exposure times (≥ 50 hours) and moderately strong sources (∼ 10% of Crab Nebula flux).