Astronomers locate source of high-energy cosmic rays


About a century ago, scientists began to realize that some of the radiation we detect in Earth’s atmosphere is not of local origin. This ultimately led to the discovery of cosmic rays, high energy protons, and atomic nuclei that were stripped of their electrons and accelerated to relativistic speeds (close to the speed of light). However, several mysteries still surround this strange (and potentially deadly) phenomenon.

This includes questions about their origins and how the main component of cosmic rays (protons) are accelerated to such a high rate. Thanks to new research from Nagoya University, scientists have for the first time quantified the amount of cosmic rays produced in a supernova remnant. This research has helped solve a 100-year-old mystery and is a major step towards precisely determining where cosmic rays are coming from.

While scientists theorize that cosmic rays come from many sources – our Sun, supernovae, gamma-ray bursts (GRBs), and active galactic nuclei (aka quasars) – their exact origin has been a mystery since their discovery in 1912. Likewise , astronomers have speculated that supernova remnants (the aftermath of supernova explosions) are responsible for their acceleration to almost the speed of light.

Showers of high-energy particles occur when energetic cosmic rays strike the top of Earth’s atmosphere. Cosmic rays were unexpectedly discovered in 1912. Illustration credit: Simon Swordy (U. Chicago), NASA.

As they travel through our galaxy, cosmic rays play a role in the chemical evolution of the interstellar medium (ISM). As such, understanding their origin is essential to understanding how galaxies evolve. In recent years, better observations have led some scientists to speculate that supernova remnants give rise to cosmic rays because the protons they accelerate interact with ISM protons to create very high energy gamma rays ( VHE).

However, gamma rays are also produced by electrons that interact with photons in the ISM, which can be in the form of infrared photons or diffuse cosmic background (CMB) radiation. Therefore, determining which source is the greatest is paramount in determining the origin of cosmic rays. Hoping to shed some light on this point, the research team – which included members of Nagoya University, the National Astronomical Observatory of Japan (NAOJ) and the University of Adelaide, Australia – observed the rest of the supernova RX J1713.7–3946 (RX J1713).

Key to their research was the new approach they developed to quantify the source of gamma rays in interstellar space. Previous observations have shown that the intensity of HEV gamma rays caused by the collision of protons with other protons in the ISM is proportional to the density of interstellar gas, which is discernible using radio imaging. On the other hand, the gamma rays caused by the interaction of electrons with photons in the ISM should also be proportional to the intensity of the non-thermal x-rays of the electrons.

For the purposes of their study, the team relied on data obtained by the High Energy Stereoscopic System (HESS), a HEV gamma ray observatory located in Namibia (and operated by the Max Planck Institute for Nuclear Physics) . They then combined this data with X-ray data obtained by ESA’s Multi-Mirror X-Ray Mission (XMM-Newton) observatory and data on the distribution of gas in the interstellar medium.

Cosmic rays produced by gamma rays relative to electrons (top), and data obtained from HESS and XMM-Newton observations (bottom). Credit: Astrophysics Laboratory / Nagoya University

They then combined the three sets of data and determined that protons make up 67 ± 8% of cosmic rays while electrons from cosmic rays make up 33 ± 8% – roughly a 70/30 split. These discoveries are revolutionary because it is the first time that the possible origins of cosmic rays have been quantified. They are also the most definitive evidence to date that supernova remnants are the source of cosmic rays.

These results also demonstrate that proton gamma rays are more frequent in gas-rich interstellar regions, while those caused by electrons are enhanced in gas-poor regions. This confirms what many researchers have predicted, namely that the two mechanisms work together to influence the course of ISM. Noted Professor Emeritus Yasuo Fukui, who was the lead author of the study:

“This new method could not have been achieved without international collaborations. [It] will be applied to more supernova remnants using the next-generation CTA (Cherenkov Telescope Array) gamma-ray telescope in addition to existing observatories, which will significantly advance the study of the origin of cosmic rays.

In addition to leading this project, Fukui has been working to quantify the distribution of interstellar gas since 2003 using the NANTEN radio telescope Las Campanas Observatory in Chile and Compact array of Australian telescopes. Thanks to Professor Gavin Rowell and Dr Sabrina Einecke of the University of Adelaide (co-authors of the study) and the HESS team, the spatial resolution and sensitivity of gamma observatories has finally reached the point where it is. possible to make comparisons. between the two.

Meanwhile, NAOJ co-author Dr Hidetoshi Sano led the analysis of the archival datasets from the XMM-Newton Observatory. In this regard, this study also shows how international collaborations and data sharing enable all kinds of cutting-edge research. With improved instruments, improved methods and greater opportunities for cooperation, we are leading into an age where astronomical breakthroughs are becoming a regular occurrence!

Further reading: Nagoya University, The Journal of Astrophysics

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