H.E.S.S. researchers observe time-dependent particle acceleration outside our solar system for the first time
Novae are powerful eruptions on the surface of a white dwarf in a binary star system, in which a larger star and a smaller star orbit each other. A nova creates a shock wave that tears through the surrounding medium, pulling particles with it and accelerating them to extreme energies. Researchers from the H.E.S.S. facility in Namibia have now been able to observe the acceleration process for the first time. Surprisingly, the RS Ophiuchi nova seems to cause particles to accelerate at speeds reaching the theoretical limit, corresponding to ideal conditions.
Novae events occur when the white dwarf gathers material from its massive companion star due to its gravity. Some novae are known to repeat. These recurrent novae are the result of thermo-nuclear explosions on the surface of old, very compact stars known as white dwarfs. RS Ophiuchi is one of these recurrent novae. There is an explosion on its surface every 15 to 20 years. “The stars forming the system are at approximately the same distance from each other as the Earth and the Sun,” explains Alison Mitchell, researcher at Friedrich-Alexander-Universität Erlangen-Nürnberg and principal investigator of the H.E.S.S Nova programme. “When the nova exploded in August 2021, the H.E.S.S. telescopes allowed us to observe a galactic explosion in very-high-energy gamma rays for the first time,” she continues. UNAM graduate and (meanwhile) H.E.S.S. observatory staff Sennae Kankondi, as well as UNAM physics student Sigrid Shilunga, were part of the observing team operating the telescopes during the observations.
The researchers observed that the particles were accelerated to energies several hundreds of times higher than previously observed in novae. Additionally, the energy released as a result of the explosion was transformed extremely efficiently into accelerated protons and heavy nuclei, such that the particle acceleration reached the maximum speeds calculated in theoretical models. According to Ruslan Konno, one of the lead authors of the study and a doctoral candidate at DESY in Zeuthen, “The observation that the theoretical limit for particle acceleration can actually be reached in genuine cosmic shock waves has enormous implications for astrophysics. It suggests that the acceleration process could be just as efficient in their much more extreme relatives, supernovae.”
During the eruption of RS Ophiuchi, the researchers were able for the first time to follow the development of the nova in real-time, allowing them to observe and study cosmic particle acceleration as if they were watching a movie. The researchers were able to measure high-energy gamma rays up to one month after the explosion. “This is the first time we have ever been able to carry out observations like this, and it will allow us to gain even more accurate future insights into how cosmic explosions work,” explains Dmitry Khangulyan, a theoretical astrophysicist at Rikkyo University in Tokyo, Japan. “We may, for example, discover that novae contribute to the ever-present sea of cosmic rays and therefore have a considerable effect on the dynamics of their immediate surroundings.”
Specific telescopes were required for these measurements. The H.E.S.S. facility (which stands for High Energy Stereoscopic System), consists of five Cherenkov telescopes that are used to investigate gamma rays from space. A new, highly sensitive state-of-the-art camera – known as FlashCam – was recently installed in the largest telescope. The FlashCam design is currently being further developed for the next generation gamma-ray observatory, the Cherenkov Telescope Array (CTA). “The new camera has been in use since late 2019, and this measurement shows just how much potential the latest generation of cameras has,” explains Simon Steinmassl, a doctoral candidate at the Max Planck Institute for Nuclear Physics in Heidelberg, who was involved in analysing the camera data.
The telescopes were pointed towards the nova at very short notice after amateur astronomers first reported the nova to the astrophysics community. The success of the observation was due in no small part to the rapid reaction of the researchers and the wider astronomical community, paving the way for extensive subsequent observations. Stefan Wagner, a professor at the regional observatory in Heidelberg and the director of the H.E.S.S. experiment, explains, “over the next few years, research using the CTA telescopes will show whether this type of nova is special.” In addition, researchers now have a clearer idea of what to look for. This gives rise to a number of new possibilities for gaining a better understanding and being better able to explain events linked to novae. “This measurement is a further breakthrough in gamma-ray astronomy and an encouraging sign that we will be able to study many more cosmic explosions with H.E.S.S. and gamma-ray telescopes of the future.” Michael Backes, Associate Professor and head of the Namibian H.E.S.S. group at UNAM adds, “Being able to succeed with such ground-breaking discoveries even 20 years into the operation speaks volume about the capabilities of the H.E.S.S. telescopes and gives prospects of many more discoveries in the years to come.”
About H.E.S.S.
More than 230 scientists from 41 institutes in 15 countries (Namibia, South Africa, Germany, France, the UK, Ireland, Italy, Austria, the Netherlands, Poland, Sweden, Armenia, Japan, China, and Australia) are involved in the international H.E.S.S. collaboration and have contributed to this research. H.E.S.S. is a system of five Imaging Atmospheric Cherenkov Telescopes for studying cosmic gamma rays. The name H.E.S.S. stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Franz Hess, who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. The H.E.S.S. telescopes are located in Namibia, near the Gamsberg Mountain, an area well known for its excellent optical properties. Four H.E.S.S. telescopes went into operation in 2002/2003, the much larger fifth telescope known as H.E.S.S. II is operational since July 2012 and extending the energy coverage towards lower energies, as well as further improving sensitivity.
The University of Namibia (UNAM) is a member of the H.E.S.S. collaboration since its inception. Currently, there are 7 staff members, 4 with PhD and 3 working towards it, and 3 MSc and Honours students conducting their research in the context of H.E.S.S. and gamma-ray astronomy in the Faculty of Agriculture, Engineering and Natural Science, spearheaded by the Department of Physics, Chemistry and Material Science at UNAM. During the past 3 years, the activities of this group were supported by a grant from the UK Global Challenges Research Fund together with the University of Oxford. The support for observations by the Namibian local crew and UNAM students were pivotal to H.E.S.S. being able to continue observations throughout the pandemic.
Reference:
Time-resolved hadronic particle acceleration in the recurrent nova RS Ophiuchi H.E.S.S. collaboration; Science, 2022; DOI: 10.1126/science.abn0567
https://www.science.org/doi/10.1126/science.abn0567
Science contact:
– Dr Alison Mitchell, Lehrstuhl für Physik, Universität Erlangen Nürnberg, Tel.: +49 9131/85-28961, alison.mw.mitchell@fau.de
Media contact:
– Prof. Michael Backes, head of the Namibian H.E.S.S. group, Department of Physics, Chemistry and Material Science, University of Namibia, Tel.: +264 81-361 8228, mbackes@unam.na.
