When a white dwarf sucks too much material from a companion star, it novas — a thermonuclear explosion in its envelope. Now, for the first time, astronomers have succeeded in capturing the high-energy gamma rays released in the process and in following the events during such a nova for a month. The observations revealed, among other things, that the shock wave from such an explosion accelerates particles close to the theoretically achievable maximum. According to this, nova bursts are similarly efficient cosmic accelerators as supernovae and thus possible co-causes of cosmic radiation.
Unlike a supernova, in which a massive star explodes and collapses, a nova does not destroy its originator. It is triggered when a White Star – a burnt out stellar remnant – orbits in a binary star system and siphons material from its companion star. As a result, more and more gas collects around the white dwarf and it increases in mass. If this exceeds a critical limit, the so-called Chandrasekhar limit, a thermonuclear explosion occurs: Nuclear fusion processes of hydrogen set in briefly in the shell, releasing strong radiation and energy. In the resulting nova, the white dwarf ejects large parts of its “stolen” envelope, but remains unharmed. This cycle of accumulation of material and nova explosion can thus be repeated periodically.
White dwarf and red giant
Astronomers from the HESS collaboration, led by Alison Mitchell from the University of Erlangen-Nuremberg and Stefan Ohm from the German Electron Synchrotron DESY, have now, for the first time, succeeded in observing the energetic gamma-ray emission of such a nova almost from the beginning and for a whole month pursue. This was made possible by a nova outburst that occurred on August 8, 2021 in the binary star system RS Ophiuchus. Located about 7,500 light-years away in the constellation of the Ophiuchus, this binary system consists of a white dwarf and a red giant orbiting each other at a distance of just 1.48 astronomical units – almost one and a half times the distance between Earth and the Sun.
Because the white dwarf is almost continuously withdrawing material from its companion, nova bursts occur again and again in this system, some of which are even visible to the naked eye. Eight such explosions 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 at the Fermi Gamma-ray Satellite and the High Energy Stereoscopic System (HESS) in Namibia were also alerted. The latter consists of five Cherenkov telescopes that detect the tiny flashes of light produced by high-energy gamma rays when they collide with particles in the Earth’s atmosphere. The following night, the astronomers of the HESS collaboration pointed their telescopes in the direction of the newly discovered nova. Supplemented by the observation data from NASA’s Fermi space telescope, which covers the slightly less energetic gamma-ray range, the astronomers were able to follow the development of the nova for over a month.
Cosmic Particle Accelerator
“This is the first observation of a nova in very high-energy gamma-ray light,” says Alison Mitchell. The data obtained reveals that the explosion at RS Ophiuchi released large amounts of high-energy gamma rays. To reach such an intensity, protons and other particles produced by the nova must be accelerated to energies several hundred times higher than in previously observed novas. “The most likely scenario is that protons and other atomic nuclei will be sharply accelerated on the expanding blast shock front and collide with compressed material from the stellar wind that the red giant is blowing into space. This releases gamma rays,” explains co-author Brian Reville from the Max Planck Institute for Nuclear Physics. Using optical data, the team estimates that RS Ophiuchi’s shock front hurtled out into space at several thousand kilometers per second.
The observations also suggest that the energy of the explosion was most efficiently converted into the acceleration of particles in this nova burst. Protons and other particles have been accelerated to energy levels close to the theoretical maximum, astronomers report. “The observation that the theoretical limit of particle acceleration can be reached in reality in cosmic shock waves has enormous implications for astrophysics,” says co-author Ruslan Konno from DESY in Zeuthen. “It suggests that the acceleration process could be just as efficient in much more extreme cosmic explosions – the so-called supernovae.” This means that novae could also contribute more than previously thought to cosmic rays – a ubiquitous “rain” of subatomic particles that come from all directions of the cosmos. Which sources feed this radiation has only been partially clarified so far.
Source: HESS Collaboration, Science, doi: 10.1126/science.abn0567