Accelerator experiment sheds light on missing blazar radiation
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New experiments at CERN by an international team have ruled out a potential source of intergalactic magnetic fields. The existence of such fields is invoked to explain why we do not observe secondary gamma rays originating from blazars.
Led by Charles Arrowsmith at the University of Oxford, the team suggests that the absence of gamma rays could result from an unexplained phenomenon that occurred in the early universe.
A blazar is an extraordinarily bright object with a supermassive black hole at its core. Some of the matter falling into the black hole is accelerated outwards in a pair of opposing jets, creating intense beams of radiation. If a blazar jet points towards Earth, we observe a bright source of light, including high-energy gamma rays in the teraelectronvolt range.
During their journey across intergalactic space, these gamma-ray photons will occasionally collide with the background starlight that permeates the universe. These collisions can create cascades of electrons and positrons that can then scatter off photons to generate gamma rays in the gigaelectronvolt energy range. These gamma-rays should travel in the direction of the original jet, but this secondary radiation has never been detected.
Deflecting field
Magnetic fields could be the reason for this dearth, as Arrowsmith explains: “The electrons and positrons in the pair cascade would be deflected by an intergalactic magnetic field, so if this is strong enough, we could expect these pairs to be steered away from the line of sight to the blazar, along with the reprocessed gigaelectronvolt gamma rays.” It is not clear, however, that such fields exist – and if they do, what could have created them.
Another explanation for the missing gamma rays involves the extremely sparse plasma that permeates intergalactic space. The beam of electron–positron pairs could interact with this plasma, generating magnetic fields that separate the pairs. Over millions of years of travel, this process could lead to beam–plasma instabilities that reduce the beam’s ability to produce gigaelectronvolt gamma rays focused on Earth.
Oxford’s Gianluca Gregori explains, “We created an experimental platform at the HiRadMat facility at CERN to create electron–positron pairs and transport them through a metre-long ambient argon plasma, mimicking the interaction of pair cascades from blazars with the intergalactic medium”. Once the pairs had passed through the plasma, the team measured the extent of their separation.
Tightly focused
In the experiment called Fireball, the beams remained far more tightly focused than expected. “When these laboratory results are scaled up to the astrophysical system, they confirm that beam–plasma instabilities are not strong enough to explain the absence of the gigaelectronvolt gamma rays from blazars,” Arrowsmith explains. Unless the pair beam is perfectly collimated or composed of pairs with exactly equal energies, instabilities were actively suppressed in the plasma.
While the experiment suggests that an intergalactic magnetic field remains the best explanation for the lack of gamma rays, the mystery is far from solved. Gregori explains, “The early universe is believed to be extremely uniform – but magnetic fields require electric currents, which in turn need gradients and inhomogeneities in the primordial plasma.” As a result, confirming the existence of such a field could indicate new physics beyond the Standard Model that may have dominated in the early universe.
More information may become available with the opening of the Cherenkov Telescope Array Observatory. This will comprise ground-based gamma-ray detectors deployed across facilities in Spain and Chile, which will substantially improve the resolution of current-generation detectors.
The research is described in PNAS.
Sam Jarman is a science writer based in the UK
from physicsworld.com 12/12/2025
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