Unveiling the origin of the high energy emission at the center of our Galaxy and its possible connection with the past activity of SgrA*

The center of our Galaxy hosts a Super-Massive Black Hole (SMBH). Since it has been argued that the SMBH can accelerate particles up to very high energies, its current and past activity must contribute to the population of Galactic cosmic-rays (CRs), the energetic particles that pervade the Galaxy. Ten years ago the HESS experiment has reported a bright TeV point-like source at a position compatible with that of the SMBH. After subtraction of this point source, a diffuse emission stretching over few degrees along the Galactic longitude was revealed. Its relative correlation with the local gas density suggesting the emission is due to the collision of multi-TeV protons and ions with the dense clouds of interstellar gas and its hard spectrum, revealed an over-abundance of freshly injected CRs. The origin of this excess remains unclear. The objective of this thesis will be to determine the location, density, nature, propagation as well as the origin of the CR excess and test its possible connection to the past activity of the SMBH and its local environment in the Central Molecular Zone (CMZ).  We propose to study the diffuse emission to characterize the CR distribution over the whole central 200 pc as well as in the vicinity of the SMBH or possibly in the halo. Understanding how CR are accelerated and how they propagate into the CMZ is fundamental to understand if a strong outflow from the central regions powered by the intense star formation and supernova activity is enough to explain how the Fermi bubbles are powered, or if one has to rely on a past huge energy output from the SMBH.      

Proposed work: A detailed characterization of the high energy diffuse emission in the central 200 pc of our Galaxy.

The origin of the CR overabundance in the central 200 pc as well as the connection with the large Fermi bubbles will be tested thanks to a precise study of the diffuse emission. 250 h of data observations of this region have been taken by HESS between 2004 and 2013: they have already revealed many morphological characteristics as well as the presence of high energy particles up to the PeV, but were not enough to constrain significantly the evolution of morphology with energy. Recently the Galactic Center has been re-observed by HESS for 200h : these data are now available to be analyzed, as well as new powerful analysis tools recently developed within the Gamma-pi python framework, at APC and within the HESS and CTA collaborations. This analysis will provide new spectral insights on the distribution of CR in different regions of the CMZ, particularly at large longitude and close to the SMBH especially at high energy. In parallel, investigations to look for possible local accelerators in the residuals and search for associated X-ray signatures in order to determine their nature will be held. After removing all disk contributions, a search for lower surface brightness emission at |b|>0.3° that will be associated with an outflow of CR from the GC as discussed by Crocker (2012) should be performed. A dedicated re-analysis of the Fermi High energy map of the region could be performed in order to make the connection with lower energy cosmic rays and the birth of the Fermi Bubbles.



Arache Djannati-Ataï & Anne Lemière






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