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 years, 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 turn 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 and Radio) 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 main difficulty lies in disentangling the origin of the gamma-ray emission, which can come from hadronic as well as from leptonic processes, namely Inverse Compton scattering of high energy electrons on the radiation background. Our group has put much effort in the detailed modeling of the multi-wavelength emission of a few of these remnants to the aim of clarifying

The physical conditions and processes in SNR shocks are, however, far from being assessed. For instance, our group has 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 the accelerated particles. A very good diagnostics of this neutral component is provided by the so-called "non-radiative" Balmer emission, in optical. Combining information coming from observations in this optical line with measurements in the X-ray band will eventually allow us to constrain the structure of the shock, and its CR acceleration 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, the theoretical modeling of their injection into the acceleration process has only recently started. Our group has deeply been involved in the study of the process efficiency from the observations’ point of view. There is indeed observational evidence that the efficiency in accelerating electrons is a smooth function of some global parameters of the SNR. This is proven by well-known correlations, 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.