Cassiopeia A is a famous SNR, the product of a gigantic explosion of a massive star that happened about 350 years ago. Discovered by radio observations 50 years ago, now we know that its emitted radiation spans from radio to high-energy gamma rays. It is also one of the few SNRs for which the birth date and the type of supernova (type IIb, the result of a core collapse supernova explosion) are known. The precise knowledge of its nature makes Cassiopeia A one of the most interesting and investigated objects in the sky, and in particular the study of its connection with the cosmic rays.
The very-high-energy part of the spectrum of Cassiopeia A results from the cosmic rays (either electrons or protons) interacting within the remnant. Until now, this range of energy could not be measured with sufficient precision to pinpoint its origin. Sensitive observations above 1 TeV were required but achieving them was daunting. The MAGIC Collaboration has finally succeeded in performing those observations. More than 160 hours of data were recorded between December 2014 and October 2016, revealing that Cassiopeia A is an accelerator of massive particles, mostly hydrogen nuclei (protons). However, even though those particles are 100 times more energetics than the ones we can reach in artificial accelerators such the one in CERN, their energy is not high enough to fully explain spectrum of the cosmic ray sea that fills our Galaxy.
Cassiopeia A was considered the perfect object to be a PeVatron, that is, an accelerator of cosmic rays up to PeV energies: it is young, bright, with a shock expanding a great velocity and with very large magnetic fields that can accelerate cosmic rays up to at least, conservatively, 100 or 200 TeV. But contrary to what we expected, in Cassiopeia A the particle energies do not reach more than a few tens of TeV. At these energies, the radiation suddenly drops and the emission stops abruptly (see Figure 2): either the remnant cannot accelerate the particles to higher energies, which challenges our knowledge of shock acceleration, or maybe, the fastest ones escaped quickly the shock, leaving only the slowest ones for us to observe.
Thanks to MAGIC observations, now we know that Cassiopeia A is accelerating cosmic rays, although to a rather low energy of a few TeV. Some of the observed gamma rays could be caused by accelerated electrons, rather than cosmic rays, but that would not affect the conclusion that acceleration in Cassiopeia A falls short of the maximum energies of the Galactic cosmic ray spectrum.
A detailed study of the observed spectral cut-off shape is crucial to understand the reason behind this low acceleration efficiency, because it displays different characteristics if due to escape of cosmic rays, to the maximum acceleration energy, or some other mechanism. Observations with the future Cherenkov Telescope Array, with a superior angular resolution and sensitivity, will allow a detailed spectroscopic investigation on the cut-off regime, providing new insights on the acceleration processes in Cassiopeia A and SNRs in general.