2025-11-04 オックスフォード大学
In the heart of an active galaxy, matter falling toward a supermassive black hole creates jets of particles traveling near the speed of light. For active galaxies classified as blazars, one of these jets beams almost directly toward Earth. Image credit: NASA/Goddard Space Flight Center Conceptual Image Lab.
＜関連情報＞
- https://www.ox.ac.uk/news/2025-11-04-scientists-recreate-cosmic-fireballs-probe-mystery-missing-gamma-rays
- https://www.pnas.org/doi/10.1073/pnas.2513365122
ブレーザー対カスケードの実験室類似体における対ビーム不安定性の抑制 Suppression of pair beam instabilities in a laboratory analogue of blazar pair cascades
Charles D. Arrowsmith, Francesco Miniati, Pablo J. Bilbao, +27 , and Gianluca Gregori
Proceedings of the National Academy of Sciences Published:November 7, 2025
DOI:https://doi.org/10.1073/pnas.2513365122
Significance
In this work, a dense beam of electron–positron pairs is produced using protons accelerated by the Super Proton Synchrotron at CERN. The beam is propagated through an ambient plasma, analogous to pair cascades produced as blazar jets propagate through the intergalactic medium (IGM). It has been proposed that plasma instabilities disrupt these pair cascades, explaining the lack of secondary γ-rays observed from blazars. However, we find that under nonideal conditions likely to be relevant in the blazar context, pair beam instabilities are strongly suppressed and it is unlikely they play a significant role. This experimental study supports the hypothesis that the IGM contains a magnetic field of unknown origin that may well be a relic of the early Universe.
Abstract
The generation of dense electron–positron pair beams in the laboratory can enable direct tests of theoretical models of γ-ray bursts and active galactic nuclei. We have successfully achieved this using ultrarelativistic protons accelerated by the Super Proton Synchrotron at (CERN). In the first application of this experimental platform, the stability of the pair beam is studied as it propagates through a meter-length plasma, analogous to TeV γ-ray-induced pair cascades in the intergalactic medium. It has been argued that pair beam instabilities disrupt the cascade, thus accounting for the observed lack of reprocessed GeV emission from TeV blazars. If true, this would remove the need for a moderate strength intergalactic magnetic field to explain the observations. We find that the pair beam instability is suppressed if the beam is not perfectly collimated or monochromatic, hence the lower limit to the intergalactic magnetic field inferred from γ-ray observations of blazars is robust.
