A new star as a particle accelerator: the explosion of a white dwarf emits a surprising amount of high-energy gamma radiation.

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Fusion shock: For the first time, astronomers have been able to track a new white dwarf star in the high-energy gamma-ray range. This showed that the explosion on RS Ophiuchi, 7500 light-years away, released enough energy to accelerate subatomic particles close to their theoretical maximum. Thus, novae are efficient cosmic accelerators and thus a possible cause of cosmic rays, the team reports in Science.

A nova occurs when a white dwarf in a binary star systemovereats“: It sucks matter from its companion star until its mass exceeds the critical limit, the Chandrasekhar limit. As a result, a thermonuclear chain reaction occurs in its inflated shell and it explodes. Such a nova emitted a large amount of radiation and maybe even in the sky naked eye become visible.

Unlike supernova massive stars, a white dwarf persists as a nova. Therefore, she can continue her stellar cannibalism – until the next new one.

RS OphiuchusIn the RS Ophiuchi system, a white dwarf is sucking material out of a red giant. © DESY/HESS, Scientific Communication Laboratory

white dwarf explodes

This is the first time astronomers have observed such a nova at gamma-ray wavelengths. This was made possible by the outburst of a new star in the binary star system RS Ophiuchi, located at a distance of 7500 light years. In it, a white dwarf and a red giant revolve around each other at a distance of almost one and a half times greater than the distance between the Earth and the Sun. Since the white dwarf constantly draws material from its companion, new ones appear again and again – between 1889 and 2006 alone there were eight.

On August 8, 2021, optical telescopes again recorded such a nova flare at RS Ophiuchi. Astronomers around the world were immediately alerted, including researchers at the HESS observatory in Namibia and NASA’s Fermi Space Telescope team. This made it possible to follow the progress of the cosmic explosion by the emitted gamma rays for a month.

Gamma rays of high energy and intensity

“This is the first observation of a new star in very high-energy gamma rays,” says Alison Mitchell of the University of Erlangen-Nuremberg, head of the HESS nova program. The type and intensity of this high-energy radiation now provides valuable information about what is happening on the white dwarf. Among other things, observations confirm that the nova also emits gamma rays and can reach surprisingly high intensity.

As astronomers have determined, the gamma radiation of the Ophiuchus RS has reached high intensities up to the teraelectronvolt range. The telescopes first registered an increase in lower-energy gamma rays, and then higher energies followed with a delay of about two days. In general, the amount of gamma radiation released was so great that this new star should have accelerated protons and other particles to energies hundreds of times higher than previously thought for such explosions.

gamma spectrumEnergy spectrum of gamma rays measured by Fermi and HESS on RS Ophiuchi. © HESS Collaboration

Carried away by the shock front

But where does this radiation come from? “The most likely scenario is that protons and other atomic nuclei will be rapidly accelerated on the expanding shock wave front and collide with the compressed matter of the stellar wind, which is blown into space by the red giant. This releases gamma rays,” explains co-author Brian Revill of the Institute of Nuclear Physics. Max Planck.

Using optical data, the team calculated that the RS Ophiuchi shock front rushed into space at a speed of several thousand kilometers per second. In total, this new one could release energy of 1043 ergs – the equivalent of 0.3 masses of terrestrial matter accelerated to 4000 kilometers per second.

Close to theoretical maximum

What surprised: the particles of matter ejected by the new, apparently, were accelerated to values ​​close to the theoretical maximum. “The observation that the theoretical limit of particle acceleration can be reached in cosmic shock waves has huge implications for astrophysics,” says co-author Ruslan Konno of DESY in Zeuthen. “This suggests that the acceleration process could be just as effective in much more extreme cosmic explosions – so-called supernovae.”

The most interesting thing about this is that it means that new stars can also contribute more to cosmic rays than previously thought. It is present throughout the universe and is made up of high-energy, fast-moving subatomic particles. However, the sources of this ubiquitous radiation are only partially elucidated. New observations now show that explosions on white dwarfs may also contribute.

For RS Ophiuchi, the astronomers calculated: “Each new event produces enough cosmic rays to fill a cube of one cubic parsec with an energy density of 0.1 electron volts per cubic centimeter — the same order of magnitude as supernovae. cosmic rays are radiation in our local galactic environment,” the team said. A cubic parsec corresponds to a cube with an edge length of about 3.2 light years. (Science, 2022; doi:10.1126/science.abn0567)

Source: Max Planck Society, German Electron Synchrotron DESY.

March 11, 2022

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