DIFFUSEGAMMAEMISSION · Sources of cosmic rays and gamma ray emission from the Galaxy

Sources of cosmic rays and gamma ray emission from the Galaxy

The ultimate objective of the proposed research project was to study the distribution of cosmic rays (CRs) and CR sources in the Galaxy, by comparing the high spatial resolution gamma ray maps, becoming available now from state of the art instruments, and atomic radio and molecular sub millimetre observations tracing the ISM gas density. The study has been carried out in regions of high gas density, so called molecular clouds (MCs), located close to the candidate CR sources, supernova remnants (SNRs), and in MCs far away from such CR sources. Molecular clouds are expected to emit non-thermal radiation due to CR interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of cosmic rays and if energetic particles can leave the accelerator site and diffusively reach the cloud. We calculated the multi wavelength spectrum from radio to gamma rays which is emerging from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma-ray emission, which can exceed the emission in other bands by an order of magnitude or more. This suggests that some of the unidentified sources detected at TeV (=1012 eV) energies so far, with no obvious or very weak counterparts in other wavelengths, might be in fact associated with clouds illuminated by cosmic rays coming from a nearby source. Moreover, under certain conditions, the gamma-ray spectrum exhibits a concave shape, being steep at low energies and hard at high energies. This fact might have important implications for the studies of the spectral compatibility of GeV (=109 eV) and TeV gamma-ray sources. Using real gas data from the surveys of molecular and atomic hydrogen in the Galaxy we also modelled the GeV to TeV gamma-ray emission produced by the collisions of runaway cosmic rays with the gas in the environment surrounding the shell-type supernova remnant RX J1713.7-3946. We studied in detail the spectral and spatial distributions of the emission, which depend upon the source age, the source injection history, the diffusion regime and the distribution of the ambient gas. In particular, we found for the region surrounding RX J1713-3946 that depending on the energy one is observing at, one may observe startlingly different spectra or may not detect any enhanced emission with respect to the diffuse emission contributed by background cosmic rays. This result has important implications for current and future gamma-ray experiments, such as Fermi and Cherenkov Telescope Array (CTA), in order to test the paradigm according to which diffusive shock acceleration in supernova remnants can explain the Galactic CR spectrum (Gabici et al., 2009, Casanova et al., 2010a). The emissivity of MCs located far away from CR sources, so called passive MCs, i.e. clouds which are illuminated by the supposedly existing CR background, can be used to probe the level of this CR background, also called CR sea. Given that the gamma-ray-emission from the MC depends only upon the total mass of the cloud, M, and its distance from the Earth, d, the CR flux in the cloud is uniquely determined as the ratio of the integral gamma-ray flux from the cloud multiplied by the distance square and divided by the mass of the cloud. Under the assumption that the CR background is equal to the locally observed CR flux, the calculated gamma-ray flux from the cloud can be compared to the observed gamma-ray flux in order to probe the CR spectrum in distant regions of the Galaxy. The detection of under-luminous clouds with the respect to predictions based on the CR flux at Earth would suggest that the local CR density is enhanced with respect to the Galactic average density. This would cast doubts on the assumption that the local CRs are produced only by distant sources, and that the CR flux and spectrum measured locally is representative of the typical CR flux and spectrum present throughout the Galaxy.

Data: CORDIS, © European Union