Cassiopeia A is a famous supernova remnant, the product of a gigantic explosion of a massive star about 350 years ago. Although discovered in radio observations 50 years ago, we now know that its emitted radiation spans from radio through high-energy gamma rays. It is also one of the few remnants for which the birth date and the type of supernova are known. It was a type IIb, the result of a core collapse supernova explosion. 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 cosmic rays, subatomic particles that fill the galaxy with energies higher than anything achievable in laboratories on Earth.
The very high-energy part of the spectrum of Cassiopeia A results from cosmic rays (either electrons or protons) within the remnant. Until now, this range of energy could not be measured with sufficient precision to pinpoint its origin. Sensitive observations above 1 Tera-electronvolts (TeV) were required, but achieving them was daunting. An international team led by scientists from the Institute for Space Sciences and collaborators has finally achieved such observations with the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope. The researchers recorded more than 160 hours of data between December 2014 and October 2016, revealing that Cassiopeia A is an accelerator of massive particles, mostly hydrogen nuclei (protons). However, even when those particles are 100 times more energetic than those in artificial accelerators, their energy is not high enough to explain the cosmic rays that fill our galaxy.