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Probing the origin of intergalactic magnetic fields and cosmic rays with gamma-ray cascades

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Tracing weaker ‘seed’ magnetic fields roaming in Space

Our Universe is swarming with weak magnetic fields – even the sparsest and dimmer cosmic stretches, where little seems to be happening, are threaded with magnetism. EU-funded researchers are searching for evidence of their presence in high-energy light emitted from distant galaxies.

Fundamental Research
Space

Magnetic fields stretch millions of light years across the Universe. Very weak fields, around a million times weaker than Earth’s own magnetic field, can thread through and between stars in our Milky Way or act like enormous filaments linking distant galaxies. They do so by keeping quiet about their activities, except when they wrap and give rise to spectacular displays such as active galactic nuclei.

How the Universe got its primordial magnetic field

Explaining the origin of these weak magnetic fields and their amplification up to their current levels remains a great puzzle. The most plausible explanation for how stronger magnetic fields could have grown from seed fields is the dynamo effect. “The concept is analogous to mixing a small amount of fermented dough with a larger amount, which causes the mix to puff up and rise. Similarly, during galaxy formation in the early Universe, tiny seed fields ignited by the dynamo effect led to the generation of stronger magnetic fields,” explains Manuel Meyer, coordinator of the GammaRayCascades project that received funding under the Marie Skłodowska-Curie Actions programme. While this justification provides a conclusion to one mystery, it also begs the question of where did seed fields originate. Independently of their formation mechanism, they are expected to be found in void regions in the Universe. These fields could measure a billionth of a billionth of the magnetic field on Earth.

Gamma rays as probes for tiny magnetic fields

“Our research focused on observing seed magnetic fields indirectly, using gamma-ray observations from distant galaxies. Such radiation is much more energetic than X-rays and is produced by charged particles travelling close to the speed of light in plasma outflows emitted by active galactic nuclei, so-called blazars,” remarks Meyer. On their journey towards Earth, high-energy gamma rays collide with lower-energy photons from the cosmic background radiation, annihilate and create electron–positron pairs. These charged particles ‘feel’ the weak magnetic field and start to gyrate in the field. As long as there are free photons streaming through the Universe, gamma rays will continue to ‘kick’ these photons to higher energies, creating a gamma-ray cascade and electron–positron pairs. It is then important to distinguish which gamma rays stem from the cascade signal and which ones directly from the source (blazar). “Since the electrons and positrons are following curved paths in the magnetic field, the gamma rays seem to deflect from the blazar on their way to Earth, embracing this point-like blazar with a halo of radiation,” notes Meyer.

Why are seed magnetic fields so important

Combining observation data from the Fermi Large Area Telescope and HESS, researchers set unprecedented narrow limits on the low threshold that the strength of the seed field must overcome to detect this halo. Researchers also reported that observing a particular blazar with the future Cherenkov Telescope Array for 50 hours could help probe so-far unconstrained strengths of the seed magnetic fields. Their results are reported here. “Determining the strength of the seed magnetic fields could serve as input for large-scale simulations of galaxy formation. Such measurements could also help determine how strongly cosmic rays from beyond the Milky Way are deflected on their way to Earth,” notes Meyer. A similar study investigating how magnetic fields polarise the radio signals was conducted by LODESTONE. “Ultimately, our research could enlighten about whether the seed fields were created in the extremely hot primordial plasma shortly after the Big Bang or during stellar explosions when ejected gas and interior magnetic fields ‘polluted’ the rest of the Universe,” concludes Meyer.

Keywords

GammaRayCascades, gamma rays, blazar, seed magnetic fields, active galactic nuclei, galaxy formation, dynamo effect, electron–positron pairs, halo

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