When a white dwarf absorbs a lot of material from a companion star, it is a nova – a thermonuclear explosion in its atmosphere. Now, for the first time, astronomers have succeeded in capturing high-energy gamma rays emitted in the process and in tracking what happens during such a supernova for a month. Observations revealed, among other things, that the shock wave from such an explosion accelerates particles close to a theoretically achievable maximum. According to this, new eruptions are efficient cosmic accelerators such as supernovae, and thus are possible common causes of cosmic radiation.
Unlike a supernova, in which a massive star explodes and collapses, a supernova does not destroy its creator. It is triggered when a white star – a burnt-out stellar remnant – orbits in a binary star system and draws material from its companion star. As a result, more and more gases gather around the white dwarf and its mass increases. If this critical limit is exceeded, the so-called Chandrasekhar limit, a thermonuclear explosion occurs: the processes of nuclear fusion of hydrogen are briefly placed in the shell, releasing powerful radiation and energy. In the resulting supernova, the white dwarf spews out large portions of its “stolen” shell, but remains intact. This cycle of material accumulation and nova explosion can thus be repeated periodically.
White Dwarf and Red Giant
HESS astronomers, led by Alison Mitchell of the University of Erlangen-Nuremberg and Stefan Ohm of Germany’s Electron Synchrotron DESY, succeeded for the first time in observing the energetic gamma-ray emission of such a supernova almost from the start and for a month-long follow-up. This was made possible by a nova explosion that occurred on August 8, 2021 in the binary star system RS Ophiuchus. Located about 7,500 light-years away in the constellation of Ophiuchus, this duo consists of a white dwarf and a red giant that orbit each other at a distance of just 1.48 astronomical units — nearly one and a half times the distance between the Earth and the Sun.
Because a white dwarf pulls material from its companion almost continuously, nova explosions occur repeatedly in this system, some of which are visible to the naked eye. Eight such outbursts were detected between 1898 and 2006. When optical telescopes again detected an increase in brightness at RS Ophiuchi on the night of August 8, 2021, astronomers on the Fermi Gamma Ray Satellite and High Energy Stereoscopic System (HESS) in Namibia were also alerted. . The latter consists of five Cherenkov telescopes that detect tiny flashes of light produced by high-energy gamma rays when they hit particles in Earth’s atmosphere. The next night, HESS astronomers pointed their telescopes in the direction of the newly discovered supernova. Completed with observations from NASA’s Fermi Space Telescope, which covers the less active gamma-ray band, astronomers have been able to follow the evolution of the nova for more than a month.
cosmic particle accelerator
“This is the first time that a supernova has been observed in high-energy gamma-ray light,” says Alison Mitchell. The data obtained reveal that the explosion at RS Ophiuchi released large amounts of high-energy gamma rays. To reach this intensity, the protons and other particles produced by the novas must be accelerated to energies hundreds of times higher than they were in previous novas. The most likely scenario is that protons and other atomic nuclei will accelerate sharply on the expanding shock front and collide with matter compressed by the stellar winds that the red giant is blowing into space. Co-author Brian Revell of the Max Planck Institute for Nuclear Physics explains. Using the optical data, the team estimates that the RS Ophiuchi frontal impact slammed into space at several thousand kilometers per second.
The observations also indicate that the energy of the explosion was more efficiently converted to the acceleration of particles in this new explosion. Protons and other particles have been accelerated to energy levels close to their theoretical maximum, the astronomers report. “Noting that the theoretical limit for actually accelerating particles in cosmic shock waves can be reached has enormous implications for astrophysics,” says co-author Ruslan Konno of DESY in Zeuthen. “It suggests that the acceleration process could be just as effective in more extreme cosmic explosions – so-called supernovae.” This means that novas can also contribute more than previously assumed to cosmic rays – a ubiquitous “rain” of subatomic particles that come from all directions of the universe. The sources feeding this radiation have only been partially elucidated so far.
Source: HESS Collaboration, Science, doi: 10.1126/science.abn0567
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