• Cosmic-Ray
• Fermi-LAT
• Gamma-Ray
• HESS

The High Energy Stereoscopic System (H.E.S.S.) Collaboration recently reported the observation of a γ-ray diffuse emission from a small region surrounding Sagit- tarius A⋆(Sgr A⋆) corresponding to the Super Massive Black Hole (SMBH) located in the center of the Milky Way. The emitted spectrum is compatible with a single power-law of index ΓHESS16 = 2.32 ± 0.05stat ± 0.11sys and extends up to 50 TeV with no statistically significant evidence of a high-energy cutoff. This γ-ray emission is expected to be originated by the interaction of the hadronic component of cosmic rays (CR) with the dense interstellar medium (ISM) characterizing the region. Thus, the emission may be related to CR population (mainly protons) with energies up to PeV in the Galactic Center (GC) region. Moreover, that emission approximately traces the gas distribution in the Central Molecular Zone (CMZ) – a massive struc- ture rich in molecular gas that extends up to ∼ 250 pc away from the GC along the Galactic Plane (GP). The last H.E.S.S. results regarding the so-called Galactic Ridge (GR) region have shown that the data can be described with a single power-law with a photon index of ΓHESS17 = 2.28 ± 0.03stat ± 0.2syst, and the spectrum is fitted over an energy range up to 45 TeV. Also in this case no evidence of a high-energy cutoff was found in the H.E.S.S. data.
This thesis work starts from a discussion – presented in Chap. 1 – of the H.E.S.S. Collaboration results and their favorite interpretation in terms of a CR accelerator – a so called PeVatron – at the GC. It will then sketched an alternative recent interpre- tation – assuming a CR-sea which in the GC region is harder then that observed at the Earth position – that, in the rest of the thesis, will be scrutinized against H.E.S.S. and Fermi-LAT data.
In the second chapter an overview on CRs acceleration and diffusion are pre- sented, while in the third chapter an explanation of the different phases of the ISM setting in the γ-ray emission is introduced. Further, a detailed description of the morphology of inner ∼ 300 parsecs is reported.
In Chap. 4, in order to evaluate the contribution due to the large-scale CR back- ground also at lower energies, an ad-hoc analysis of Fermi-LAT data has been imple- mented, using the most updated data based on the PASS8 event reconstruction algo- rithm, and the Fermi-LAT source catalogues 3FGL and 3FHL. The analysis outputs consist in three all-sky maps of counts, exposure, and 3FGL and point-like sources fluxes respectively.
In Chap. 5 the diffuse emission spectrum, from few GeV to 300 GeV – based on Fermi-LAT data – is derived and combined with that measured by H.E.S.S. at larger energies for several regions in the CMZ complex. Moreover, those results are compared with the predictions of the PeVatron scenario as well as with that assuming a Galactic CR spectrum which hardens towards the GC. In particular the Gamma Model – which was built to reproduce the diffuse spectrum observed by Fermi-LAT in the inner Galactic Plane, and adopts the CR hardening found by PAMELA, AMS-02 and CREAM – has been considered. The different components of the γ-ray emission integrated along the line-of-sight expected for this model have been computed with the DRAGON and GammaSky numerical codes.
A further analysis has been performed to determine the luminosity-profile in the inner 300 parsecs of the of the Milky Way. H.E.S.S. data have been compared with the estimations of a 1/r behavior featuring the emission due to an accelerator located at Sgr A⋆ position or its vicinity, and the emission profile computed for the hard CR-sea scenario.
Having found that present data do not allow to decide among those scenarios yet, an explorative analysis has been then performed to investigate the perspectives of the Cherenkov Telescope Array (CTA).
In Chap. 6 will report the conclusion of the work and the future developments of the project.