Star clusters as gamma-ray emitters



Cosmic rays (CRs) are highly-energetic charged particles that permeate the Galaxy and reach the Earth from any direction. More than one century after their discovery, the origin of CRs remains mysterious, especially for what concerns the observed CRs of energy around a few Petaelectronvolts (1 PeV=1015 eV), where their energy distribution shows a softening known as the “knee” of the CR spectrum. Supernova Remnants (SNRs) have long been considered the best candidate accelerators in the Galaxy, but recent theoretical and observational arguments suggest that they cannot achieve easily energies exceeding a hundred TeV, preventing them to be identified as PeVatrons, i.e. accelerators of PeV particles. Recent theoretical efforts confirmed that in certain conditions, instead star clusters (SCs) are able to accelerate particles up to PeV energies. In order to understand if SCs are effective contributors of the Galactic CR population, the acceleration efficiency needs to be tuned on observations. Gamma-rays, produced by CRs interacting with the ambient medium, carry information about the spatial and spectral distribution of CRs near the sources. Gamma-ray observations of SCs are limited to a few objects. Yet, these objects are the primary PeVatron candidates: the Cygnus region has been detected up to 1.4 PeV,  while Westerlund 1 and 2 have been detected up to very high energies. 

We propose a comprehensive investigation of stellar clusters in gamma rays aimed at deriving their acceleration efficiency and their contribution to the Galactic CR population. The project will be carried out during a 4-months stage (M2) by a master intern student. The spatial correlation between SCs and gamma-ray emitter will be evaluated using the most recent available catalogs. For promising targets, gamma-ray observations accumulated by the Fermi-LAT detector will be used to extract the spectral energy distribution between a few hundred TeV to a few hundred GeV and will be used to make predictions for visibility at very high energies.



Stefano Gabici, Giada Peron






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