Arcetri High Energy Group

Supernova Remnants



After the explosion of a star (a "supernova", SN), the evolution of the stellar debris and its interaction with the surroundings gives rise to a number of fascinating phenomena. When talking of Supernova Remnants (SNRs), here we will refer to the nebular remnants that originate from the impact of the stellar matter (the "ejecta", hurled at high speed) against the ambient medium. The emission from these objects usually presents a limb-brightened morphology (where the interaction takes place) and are then referred to as "shell-type SNRs". In objects with an inner active pulsar, nebular remnants are also produced by the interaction of the pulsar relativistic wind with the surrounding SN ejecta. They are known as "pulsar wind nebulae" (PWNe), and are described in a separate section. The expansion of a SNR may last about one million year, before it eventually merges into the interstellar medium (ISM) and disappears. However, since older SNRs are also fainter and difficult to identify, the ages of known SNRs are shorter, typically around 10000-100000 yr. The evolution of a SNR can be subdivided into a few phases: There are two main limitations to the "classical" evolution of the SNRs, as outlined above. On one side it does not consider the possibility of asymmetries in the SN explosion, while observations show that asymmetries are rather the rule than the exception. On the other side it does not include the effects of cosmic-ray (CR) acceleration, while if SNRs are the main source of Galactic CRs, at least up to about 100-1000 TeV (the position of the so-called CR spectral "knee"), a near 10% efficiency in CR acceleration would be required. A separate section is devoted to CRs, but some aspects and implications of particle acceleration in SNRs will be also discussed here. It has been theoretically shown how ions can enter the acceleration processes, and how (through diffusive acceleration, also known as 1st order Fermi acceleration) they can reach very high energies. The process is non linear: the CR streaming in front of the shock discontinuity may amplify turbulent magnetic fields in the upstream region, which in turns are responsible for effective scattering of particles, and their subsequent acceleration. This means that the relativistic particles are not passively accelerated, but contribute themselves to the structure and properties of the shock, giving rise to the so-called "CR-modified shock". A number of predictions follow, both on the observed structure (mostly in X-rays) of the shock, and on its hadronic gamma-ray emission. Actually, this is the only way to directly observe accelerated ions in SNRs: through their collisions with the ambient ions they create pions, which then decay emitting gamma-ray photons. Young SNRs, like Tycho's SNR, RX J1713.7-3946, SN 1006, are the best targets for this kind of investigation. The physical conditions and processes in SNR shocks are, however, far from being assessed. For instance, it has been recently shown that, if a SNR shock moves through a partially neutral medium, a "neutral return flux" appears and can modify substantially the structure of the shock, with effects on the energy distribution of of the accelerated particles. A very good diagnostics of this neutral component is provided the so-called "non-radiative" Balmer emission, in optical. Combining information coming from observations in this optical line, and that from X-ray observations, will eventually allow us to constrain the structure of the shock, and its CR efficiency. While in terms of energy the acceleration of ions is the dominant process, electrons acceleration is the most relevant from a diagnostic point of view, since we can directly observe them through their non-thermal emission. SNRs are powerful non-thermal emitters, and in fact most of the Galactic SNRs have been discovered from their radio synchrotron emission. Unfortunately, even though there is a wealth of observational evidence that electrons are accelerated in SNR shocks, so far no clear theoretical modeling is available of how they can be injected into the acceleration process. Some injection mechanisms have been proposed, but this issue is still far from having been solved. On the other hand, there is observational evidence that the efficiency in accelerating electrons is a smooth function of some global parameters of the SNR. In fact correlations in such sense are known, starting from the so-called Sigma-D relation (an empirical relation between the SNR size and its surface brightness in radio) to more sophisticated statistical analyses that involve also the density of the ambient medium.