Radio-emitting supermassive black holes could be source of mystery neutrinos

Neutrinos detected by the IceCube telescope may originate from energetic supermassive black holes emitting radio waves, rather than the gamma ray-emitting galactic nuclei currently thought to be their source.

A paper published in the Journal of Experimental and Theoretical Physics has proposed a different model of neutrino emission, one that sees these abundant, but almost massless, subatomic particles generated by interactions between protons and the swirling collection of hot gas and dust around a black hole at the centre of a galaxy.

In 2013, the IceCube telescope in Antarctica picked up a radiation signal from a single muon – a particle generated when a neutrino interacts with ice. However, the origin of that neutrino, and many since, has long been a mystery.

The prevailing theory for the source of neutrinos is that they are emitted from gamma ray-emitting galactic nuclei. When protons, positively-charged subatomic particles, are flung out from these black holes at high speed, they release both gamma rays and neutrinos.

However, the directions from which neutrinos have arrived at IceCube don’t yet match up to known gamma-emitting galaxies. There’s also no evidence of clustering – neutrinos arriving in pairs or clusters from the same point in space – which should occur if they are coming from a single gamma-emitting source.

A recent paper, written by Andrii Neronov and Dmitri Semikoz, from the Astroparticle and Cosmology Laboratory at the Université de Paris, in France, instead attempted to find an explanation for recent evidence of correlation of the positions of neutrinos with known radio-brightest galactic nuclei, previously found by Plavin et al. These are supermassive black holes that launch powerful relativistic jets filled with high energy electrons that emit synchrotron radiation in the form of radio waves.

“This is strange because gamma rays are photons with energies close to those of neutrinos, but radio waves are produced by a mechanism which has nothing to do with the production of neutrinos,” says Neronov.

The researchers looked at what mechanisms might generate neutrinos from these radio-bright sources. The model they proposed is that the protons accelerated in the radio-emitting active galactic nuclei are actually interacting with the gas and matter in the accretion flow, which can extend far out from the black hole itself, and it is this interaction that is generating the neutrinos and simultaneously filling the jet with electrons that emit synchrotron radiation in the radio band.

Neronov says more neutrino discoveries from the radio-loud active galactic nuclei are needed to confirm whether they are coming from the direction of radio-emitting, rather than gamma-emitting galactic nuclei.