Remnants reveal origins of cosmic rays

In the 90 years since the discovery of cosmic rays, there have been many theories about their origin, but little experimental evidence for actual sources. Now two groups have evidence for sources of cosmic rays in two distinct energy regions – up to about 10¹⁵ eV, and ultrahigh energies around 10²⁰ eV.

The majority of cosmic rays are protons with energies less than around 10¹⁵ eV. In 1949 Enrico Fermi suggested that these particles could be accelerated in moving magnetic clouds, as in the shock waves surrounding a supernova. Direct evidence for acceleration in a supernova remnant has been found, but only for cosmic-ray electrons. Now, however, a team working with data from the CANGAROO II (Collaboration of Australia and Nippon for a GAmma Ray Observatory in the Outback) cosmic-ray telescope in Woomera, South Australia, has evidence that points to a specific supernova remnant as an accelerator of cosmic protons. CANGAROO II is a 10 m air Cerenkov telescope, which detects the electromagnetic showers created when gamma rays with energies of around 10¹² eV strike the atmosphere. The team has analysed data from one of the intrinsically brightest sources of gamma rays in our galaxy, the supernova remnant RX J1713.7-3946. They found that the measured energy spectrum does not fit with models in which the gamma rays are emitted by accelerated electrons. However, it agrees well with the assumption that the gamma rays come from the decays of neutral pions, presumably produced by the interaction of high-energy protons accelerated in the supernova remnant.

The possible sources of ultrahigh-energy cosmic rays, with energies around 10²⁰ eV, present a different problem. While undoubtedly exotic, the sources must also be relatively nearby, as otherwise the cosmic rays would lose energy through interactions with the cosmic microwave background radiation (the so-called GZK cut-off, after Greisen, Zatsepin and Kuzmin). Nearby dead quasars – or quasar remnants – which contain spinning supermassive black holes at their centre are one possibility, for which there is now some observational evidence. A team from Princeton University and NASA Goddard Space Flight Center has searched a catalogue of several thousand galaxies for those likely to be suitable quasar remnants nearer than 50 Mpc (or about 160 million light-years, the GZK cut-off for 10²⁰ eV) and found a sample of 12 candidates. They then looked for correlations with the arrival directions for high-energy events from AGASA (Akeno Giant Air Shower Array) in Japan. Their results indicate a non-random correlation between three galaxies and 34 events with energies greater than 4 x 10¹⁹ eV; for 7 events with energies greater than 10²⁰ eV, the correlation is less clear. The team does not yet know if the black holes in these galaxies are spinning – a necessary condition for the proposed cosmic-ray accelerator – but they do suggest further studies of nearby possible quasar remnants, particularly with the Auger Observatory.