Search for gamma-ray emission from supernova remnantswith the Fermi /LAT and MAGIC telescopes

This thesis is an astrophysical approach to the problem of th e Galactic cosmic rays. Cosmic rays are energetic particles that hit the Earth atmos phere from above. The discovery of its extraterrestrial origin is attributed to V ictor Hess, who conducted a series of key experiments one hundred years ago. The centenary of th e discovery has been celebrated at the time this thesis has been written. This coi ncidence motivated to perform a brief historical search of the key developments of the earl y history of the cosmic-ray physics. The current status of the field is also reviewed, foc using in composition, spectral shape and anisotropy searches. Chapter 1 ends discussing th e current knowledge of the possible acceleration sites of cosmic rays. At energies bel ow 10 15 . 5 eV, Galactic supernova remnants are believed to be the main contributors. The rest o f the thesis is devoted to the study of gamma-ray emission from some of these objects. Physical processes in supernova remnants are introduced in Chapter 2. The paradig- matic mechanism known as diffusive shock acceleration place s supernova remnants as efficient cosmic-ray accelerators. Information regarding p articles accelerated in super- nova remnants is best gathered by observing them in the gamma -ray domain. Therefore, emission processes yielding gamma rays are discussed. Mech anisms involving accelerated protons are of particular interest, as they prove the majori ty component in the cosmic rays. We focus on the possibility of having molecular clouds near supernova remnants enhancing the gamma-ray emission. Finally, a list of candid ates for being observed from the northern hemisphere is elaborated. The telescopes to which we had access to perform observation s of supernova remnants in the gamma-ray domain were Fermi /LAT and MAGIC. Both are introduced in Chapter 3. Fermi /LAT is a space telescope with all-sky coverage that provide s free access to the data through a web page. Therefore, the coverage of the objec ts of interest was granted in this energy range. MAGIC instead, is operated by an intern ational collaboration of institutions. As a member of the MAGIC collaboration, I was a ble to propose observations of some of the objects in our target list, that were accepted a nd conducted. The MAGIC telescopes themselves are described with some detail, in pa rticular the aspects regarding data analysis, in which the author had responsibilities wit hin the collaboration. The first of the individual objects to be discussed is the supe rnova remnant HB 21 (Chapter 4). The analysis of the public Fermi /LAT data lead us to claim the existence of a resolved gamma-ray source. We were able to divide the sou rce in three regions and obtain the spectrum of each of them. As a result, we found t hat there are spectral variations across the supernova remnant, that we attribute to variations in the properties of the interstellar medium. In Chapter 5 we discuss observations of the Cygnus Loop super nova remnant. Same as for HB 21, the source is resolved in the observations with Fermi /LAT, and we perform an analysis similar to that of HB 21, in which we find even more mar ked spectral variations. There are two spots in particular that present a harder spect rum, extending to energies of several GeV. It turns out that one these spots was targeted by our proposal to observe the Cygnus Loop with the MAGIC telescopes, prior to knowing t he information from Fermi /LAT, and based only in rough flux expectations based on the in terstellar medium density. Both the Cygnus Loop and HB 21 happen to be low-luminosity, ne arby supernova remnants. In contrast with these objects, the supernova rem nant W51C (discussed in Chapter 6) is the most luminous gamma-ray emitting supernov a remnant detected so far in the Galaxy. Our observations with the MAGIC telescope s extend the spectrum previously measured by the Fermi Collaboration up to energies of 5 TeV. In addition, we locate the emission region in a very particular zone of inter action between W51C and the adjacent star-forming region W51B. The rest of the supernov a remnant shell is dark in gamma rays, except for a secondary emission peak (not signifi cant by itself) coincident with a possible pulsar wind nebula. To conclude, we discuss the impact that these observations h ave in the field of high- energy emission from supernova remnants. We stress the conv enience of space-resolved observations for the study of the escape and diffusion of cosm ic rays from supernova rem- nants. We also note the existence of a population of low lumin osity supernova remnants of which Cygnus Loop and HB 21 are representatives. These low luminosity supernova remnants could be abundant in the Galaxy, and thus could have a large contribution to the cosmic rays.