Shredded to pieces Chandra X-ray Observatory Center/NASA

White dwarf stars shredded by black holes could explain showers of high-energy cosmic rays and neutrinos we see on Earth.

Cosmic rays and neutrinos are part of the rain of subatomic particles from space that bombard Earth every day. But what produces these difficult-to-detect particles? A team led by Daniel Biehl at Deutsches Elektronen-Synchrotron in Germany suggest that tidal disruption events in white dwarfs could be responsible.

“A tidal disruption event is what happens when a star gets too close to a black hole and the strong gravity tears the star apart,” says Cecilia Lunardini at Arizona State University. “Part of the debris of the destroyed star falls into the black hole, and this causes the black hole to emit energy and accelerate particles.”


In theory, the extreme energy of black hole jets is enough to disintegrate atomic nuclei in a cascading reaction that produces both high-energy neutrinos and ultra high-energy cosmic rays. The researchers suggest a single process – the disintegration of nuclei torn from white dwarves and accelerated in the jets of black holes – could simultaneously produce both kinds of subatomic particles. Julian Krolik at John Hopkins University agrees it’s a possibility.

Black hole jets

“We’re really not very good at understanding how cosmic rays are accelerated,” says Krolik. With an uncertain grasp of the mechanics behind how cosmic rays can reach such high velocities, it’s unclear if the extreme environment of black holes jets could be responsible.

Krolik explains that only a small portion of black holes produce relativistic jets, although scientists are uncertain why. Likewise, only a small portion of black holes have nearby white dwarf stars that can be torn apart to produce tidal disruption events. This means that it will take time and luck to successfully observe tidal disruption events on white dwarfs near black holes that are capable of producing jets.

Although tidal disruption events were theorised decades ago and researchers have spotted plenty of potential events, only a handful of observations are confirmed. “The rate of these events for a white dwarf is even more uncertain than the rate of events involving ordinary stars,” says Krolik.

An open question

This scant data means researchers don’t yet understand how tidal disruption events unfold, much less if they could be the mysterious source of the highest-energy cosmic rays and neutrinos.

Lunardini agrees and says this is a theoretical explanation that will only be confirmed if we see future simultaneous observations of X-rays indicative of a tidal disruption event and incoming neutrinos from the same patch of sky. Even then, other proceses may be at work that also produce these high-energy particles.

But as sky surveys get more comprehensive and particle detectors grow more sensitive, researchers will be better able to determine if tidal disruption events are responsible for the highest-energy particles. Even if they aren’t the only source, Lunardini thinks they could still contribute at least some of the particles.

“This is still a very open question,” agrees Lunardini. “It’s interesting to look for alternatives.”

ArXiv: arxiv.org/abs/1711.03555v1

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