Stellar explosions called novas produce high-energy cosmic rays, but astronomers have been unsure how energetic these can be. Observations of the repeating nova RS Ophiuchi have greatly upped the maximum, with a rare example of things in nature working as well as simplified models say they should. It could also have implications for the workings of the more famous and influential supernovas.

The word “nova” comes from an expression for a new star, because that was what pre-telescope astronomers thought they were, with stars appearing in places none had been seen before. Since then, we have realized the stars in question are old and faint, having exceeded their mainstream life, so we couldn't see them with the naked eye until their temporary revivals.

A nova occurs when material from an ordinary star falls upon a nearby white dwarf, igniting a brief thermonuclear explosion in the compact star's outer layers. Along with visible light, these release gamma rays, and a paper in Science reveals these can be very high energy indeed.

Some novas occur once and never again, or at unpredictable intervals, but others are more reliable. RS Ophiuchi explodes every 9 to 26 years. This not only means astronomers know to keep an eye on it when it's due, but has led us to study the relationships of the stars involved.

“The stars forming the system are at approximately the same distance from each other as the Earth and the Sun,” said Dr Alison Mitchell of the High Energy Stereoscopic System (H.E.S.S) Nova program in a statement. 

H.E.S.S detects the Cherenkov radiation cascade produced when high-energy rays encounter the upper atmosphere. “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,” Mitchell explained.

Like any explosion in space, novas accelerate particles to high energies, but researchers discovered particles from RS Ophiuchi had energies several hundred times greater than any previously observed from novas.

Theoretical models suggested the most powerful nova-accelerated protons and atomic nuclei would, under ideal conditions, be in the low terra electron volts – and when are conditions every ideal? Therefore, astrophysicists expected energies, in reality, would be lower, and had only found evidence of much lower-energy particles.

However, Mitchell and co-authors did indeed find 1-2.5 TeV particles – suggesting RS Ophiuchi is something of the platonic ideal of a particle accelerator, efficiently converting the energy of the explosion's shockwave to fast-moving particles. The time distribution for these particles is similar to that for gigavolt particles, confirming the nova is the source.

“The observation that the theoretical limit for particle acceleration can actually be reached in genuine cosmic shock waves has enormous implications for astrophysics,” Ruslan Konno, a doctoral student at the Deutsches Elektronen-Synchrotron, said. “It suggests that the acceleration process could be just as efficient in their much more extreme relatives, supernovae.”

The Earth is showered in cosmic rays from sources within and outside our galaxy. Although many of these sources are known, the origins of many of the cosmic rays remain unknown, and this discovery indicates nova's contribution may have been underestimated.

A year ago, the most powerful cosmic rays – almost a thousand times more energetic than the ones H.E.S.S detected – were revealed to come not from supernovas, but from star clusters. 

The authors attribute their success to the addition of new instruments to Namibia's H.E.S.S telescopes, and to early notice provided by amateur astronomers of RS Ophiuchi's latest explosion.