GAMMA · Gamma ray astronomy and the origin of galactic cosmic rays

Gamma ray astronomy and the origin of galactic cosmic rays

PROJECT OBJECTIVES Cosmic rays are charged and energetic particles that hit the Earth atmosphere from above. Their origin is still unknown. The main reason for that is the fact that they are deflected in the turbulent interstellar magnetic field, and thus their arrival directions do not point back to the sites of their production. According to the most popular scenario, cosmic rays are accelerated at supernova remnant shocks. The reason of the popularity of such hypothesis is the fact that supernova remnants can provide the power required to maintain the intensity of cosmic rays which is observed in the Galaxy. Though quite successful, this scenario still needs to be conclusively proven (or refuted). Gamma ray astronomy is probably the best tool to test the supernova remnant hypothesis for the origin of cosmic rays. This is because cosmic rays undergo hadronic interactions with the interstellar medium and produce neutral pions that in turn decay into gamma rays. Thus, observations of gamma rays from supernova remnants, or from the vicinity of supernova remnants (in this case we will deal with runaway cosmic rays that escaped the acceleration site) might reveal the presence of cosmic rays and support the scenario. The main goal of the research project was to develop gamma-ray based tests for the supernova remnant hypothesis forthe origin of cosmic rays. WORK PERFORMED The work performed in this direction can be divided in three separate lines of research: 1) predictions of the gamma-ray emission expected from supernova remnants, 2) predictions of the gamma ray emission expected from the vicinity of supernova remnants, 3) investigation of the role of future generation of instruments (such as the Cherenkov Telescope Array) in solving the problem of the origin of cosmic rays. MAIN RESULTS The main results obtained can be summarised as follows. 1) In the paper Cristofari et al (2013) we estimated the gamma ray fluxes expected from individual supernova remnants under the assumption that these objects indeed are the sources of cosmic rays. We then simulated the entire population of supernova remnants in the Galaxy and we estimated the number of such objects that we might expect to detect with a given gamma ray telescope. We compared the results with the data available from the HESS Cherenkov telescope and we find a substantial agreement with our predictions. This gives an additional and novel support to the hypothesis that supernova remnants are the sources of cosmic rays. Another paper is now in preparation (Cristofari et al. 2014) and will be focused on the role of future facilities like the Cherenkov Telescope Array, HAWC, Hi-Score, adn LHAASO. 2) In the paper Gabici et al. (2010) we studied the propagation of cosmic rays just after they escape their source and we predicted the gamma-ray emission expected if a massive molecular cloud is located in the vicinity of the cosmic ray source. We assumed isotropic propagation of cosmic rays. We then applied the results to the case of the supernova remnant W28, which is indeed surrounded by massive clouds. A comparison between gamma ray data and predictions allowed us to constrain the diffusion coefficient of cosmic rays. This is important because this quantity is very difficult to be measured or inferred from theory. In a following paper Nava & Gabici (2013) we extended the work by considering also the case of anisotropic diffusion, which is more physically motivated, at least on scales comparable with the coherence length of the magnetic field in the Galaxy. 3) In the paper Acero et al (2013) and Pedaletti et al. (2013) we investigated the role of the future gamma-ray telescope Cherenkov Telescope Array in studies related to the acceleration and propagation of cosmic rays in the Galaxy.

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