About a century ago, scientists began to realize that some of the radiation we discover in the earth’s atmosphere is not of local origin. This eventually led to the discovery of cosmic rays, high-energy protons and atomic nuclei, from which their electrons were withdrawn and accelerated to relativistic speeds (close to the speed of light). However, this strange (and potentially fatal) phenomenon still holds some secrets.
This includes questions about their formation and how the main component of cosmic rays (protons) is accelerated to such high speeds. Thanks to new research led by the University of Nagoya, scientists have for the first time quantified the amount of cosmic rays produced in a supernova remnant. This research helped solve a 100 year old mystery and is an important step in determining exactly where cosmic rays are coming from.
While scientists theorize that cosmic rays originate from many sources – our sun, supernovae, gamma-ray bursts (GRBs), and active galactic nuclei (also known as quasars) – their precise origins have been a mystery since their discovery in 1912, astronomers have theorized that Supernova remnants (the aftermath of supernova explosions) are responsible for accelerating them to almost the speed of light.
Showers of high-energy particles occur when high-energy cosmic rays hit the earth’s atmosphere. Cosmic rays were unexpectedly discovered in 1912. Photo credit: Simon Swordy (U. Chicago), NASA.
On its journey through our galaxy, cosmic rays play a role in the chemical evolution of the interstellar medium (ISM). Therefore, understanding their origins is important in order to understand how galaxies evolve. In recent years, improved observations have led some scientists to speculate that supernova remnants produce cosmic rays because the protons they accelerate interact with protons in the ISM to produce very high energy (VHE) gamma rays.
However, gamma rays are also generated by electrons that interact with photons in the ISM, which can be in the form of infrared photons or radiation from the Cosmic Microwave Background (CMB). So it is of the utmost importance to determine which source is larger in order to determine the origin of cosmic rays. Hoping to shed some light on this, the research team – which included members of Nagoya University, the National Astronomical Observatory of Japan (NAOJ), and the University of Adelaide, Australia – observed the supernova remnant RX J1713.7? 3946 (RX J1713).
The key to their research was the novel approach they developed to quantify the source of gamma rays in interstellar space. Previous observations have shown that the intensity of the VHE gamma radiation caused by the collision of protons with other protons in the ISM is proportional to the interstellar gas density seen by radioline imaging. On the other hand, it is expected that gamma rays caused by the interaction of electrons with photons in the ISM are also proportional to the intensity of nonthermal X-rays of electrons.
For their study, the team relied on data from the High Energy Stereoscopic System (HESS), a VHE gamma ray observatory in Namibia (and operated by the Max Planck Institute for Nuclear Physics). They then combined these with X-ray data from the ESA observatory X-ray Multi-Mirror Mission (XMM-Newton) and data on gas distribution in the interstellar medium.
Cosmic rays generated by gamma rays versus electrons (top) and data from the HESS and XMM Newton observations (bottom). Credit: Astrophysics Laboratory / Nagoya University
Then they combined all three sets of data and found that protons make up 67 ± 8% of cosmic rays, while cosmic electrons make up 33 ± 8% – roughly a split of 70/30. These findings are groundbreaking, as the possible origin of cosmic rays was quantified for the first time. They are also the most definitive evidence yet that supernova remnants are the source of cosmic rays.
These results also show that gamma rays from protons are more common in gas-rich interstellar regions, while those caused by electrons are amplified in gas-poor regions. This supports what many researchers have predicted, namely that the two mechanisms work together to affect the evolution of the ISM. Professor Emeritus Yasuo Fukui, the lead author of the study, said:
“This novel method would not have been possible without international cooperation. [It] With the next-generation gamma ray telescope CTA (Cherenkov Telescope Array), in addition to the existing observatories, it will be used on other supernova remnants, which will significantly advance research into the origin of cosmic rays. “
In addition to leading this project, Fukui has been working with the NANTEN radio telescope at the Las Campanas Observatory in Chile and the Australia Telescope Compact Array on the quantification of interstellar gas distribution since 2003. Thanks to Professor Gavin Rowell and Dr. Sabrina Einecke from the University of Adelaide (co-authors of the study) and the HESS team, the spatial resolution and sensitivity of gamma-ray observatories has finally reached the point where comparisons can be made between the two.
Meanwhile, co-author Dr. Hidetoshi Sano from the NAOJ analyzed archival datasets from the XMM Newton Observatory. In this respect, this study also shows how international cooperation and data exchange enable all types of cutting-edge research. Together with improved instruments, improved methods and greater opportunities for collaboration lead to an age where astronomical breakthroughs are the order of the day!
Further reading: Nagoya University, The Astrophysical Journal