About a century ago, scientists began to realize that some of the radiation they detected in Earth's atmosphere did not originate locally. This eventually led to the discovery of cosmic rays, where high-energy protons and nuclei were stripped of electrons and accelerated to relativistic speeds (close to the speed of light). However, there are still several mysteries surrounding this strange (and potential) phenomenon. This includes questions about their origins and how the main component of cosmic rays (protons) was accelerated to such a high speed. Thanks to a new study led by Nagoya University, scientists have for the first time quantified the number of cosmic rays produced in the remains of supernovae.
The study, which helped unravel a century-old mystery, is an important step toward pinpointing the source of cosmic rays.
While scientists theoretically believe that there are many sources of cosmic rays — the Sun, supernovae, gamma-ray bursts (GRBs) and active galactic nuclei (also known as quasars) — their exact origins have remained a mystery since they were first discovered in 1912. Similarly, astronomers speculate that supernova remnants are responsible for accelerating them to nearly the speed of light.
As cosmic rays travel through our galaxy, they play a role in the chemical evolution of the interstellar medium (ISM). Therefore, understanding their origins is crucial to understanding how galaxies evolve. In recent years, improved observations have led some scientists to speculate that supernova remnants produce cosmic rays because their accelerated protons interact with protons in ISM, producing extremely high-energy (VHE) gamma rays.
However, gamma rays are also produced by electrons interacting with photons in ISM, which can be in the form of infrared photons or radiation from the cosmic microwave background (CMB). Therefore, determining which source is larger is most important for determining the source of cosmic rays. To illustrate this, the team — including members from Nagoya University, the National Astronomical Observatory of Japan (NAOJ) and the University of Adelaide in Australia — observed the supernova remnant RX J1713.7−3946 (RX J1713).
Key to their research is that they developed a new method to quantify the source of gamma rays in interstellar space. Past observations have shown that the intensity of VHE gamma rays caused by proton collisions with other protons in ISM is proportional to the density of interstellar gas, which can be discerned with radio-line imaging. On the other hand, gamma rays caused by the interaction of electrons with photons in ISM are also expected to be proportional to the non-thermal X-ray intensities of electrons.
To conduct the study, the team relied on data obtained by the High Energy Stereoscopic System (HESS), a VHE gamma-ray observatory in Namibia (operated by the Max Planck Institute for Nuclear Physics). They then combined it with X-ray data obtained by the ESA X-ray Multi-Mirror Mission (XMM-Newton) observatory and data on the distribution of gas in the interstellar medium.
They then combined all three datasets to determine that protons accounted for 67±8 percent of cosmic rays, while cosmic ray electrons accounted for 33±8 percent — about a 70/30 ratio. These findings are groundbreaking because they are the first to quantify the possible sources of cosmic rays. They also constitute the clearest evidence to date that supernova remnants are a source of cosmic rays.
The results also suggest that proton-induced gamma rays are more common in gas-rich interstellar regions, while electron-induced gamma rays are enhanced in gas-poor regions. This supports many researchers' prediction that these two mechanisms together influenced the evolution of ISM. Yasuo Fukui, lead author of the study and professor emeritus, said:
"This entirely new approach would not have been possible without international cooperation. [It] will be applied to more supernova remnants, in addition to existing observatories, using the next generation of gamma-ray telescopeSCTA (Cherenkov Telescope Array), which will greatly advance the study of the origin of cosmic rays. "
In addition to leading the project, Yasuo Fukui has been working since 2003 to quantify the distribution of interstellar gas using the NANTEN Radio Telescope at the Las Campanas Observatory in Chile and the Australian Telescope Compact Array. Thanks to Professor Gavin Rowell and Dr Sabrina Einecke of the University of Adelaide (co-authors of the study) and the H.E.S.S. team, the spatial resolution and sensitivity of the gamma-ray observatory has finally reached a point where the two can be compared.
Meanwhile, co-author Dr Hidetoshi Sano of NAOJ led the analysis of the archival dataset of the XMM-Newton Observatory. In this regard, the study also shows how international collaboration and data sharing have enabled a variety of cutting-edge research. Along with improvements in instrumentation, improvements in methods and more opportunities for collaboration, it is leading to an era in which astronomical breakthroughs become frequent.