Could it be that vast magnetic structures filling as much as ten percent of the universe have remained all but invisible to us until now? That’s the startling possibility raised by Gregory Benford (UC-Irvine) and Raymond Protheroe (University of Adelaide) in a new paper describing a possible source for ultra-high energy cosmic rays. They’re looking at the remnants of jets that can be found in active galactic nuclei (AGN), and suggesting that even after these jets have turned off, a fossil structure may remain that is stable for billions of years.
What exactly is the remnant of a jet? Here’s the notion as explained in the authors’ upcoming paper:
Remnants of jets and their surrounding cocoons may persist long after their parent AGN fade from view. These colossal MHD structures decay slowly and yet may retain their relatively stable self-organized conﬁgurations. Decay depends on the structure circuit resistance, and lifetimes could be quite long, given the large inductance of the circuit, an initial outward current along the jet and a return current back along an outer sheath or cocoon around the jet.
So imagine a galactic jet forming from rotating, magnetized plasma stemming from the galaxy’s central black hole. When the jet finally switches off, the fossil structure organizes itself into a configuration known as a reversed-field pinch, where the longitudinal magnetic field changes direction at a critical distance from the axis. The configuration is stable, the decay time slow. In fact, Benford and Protheroe estimate that on the immense scale involved, a fossil jet could take billions of years to decay. And this is where we move into the realm of the observable, for the decay involved should induce huge electric fields capable of accelerating cosmic rays up to ultra-high energies.
In other words, the fossil jet structures in question may help us understand the enigma of a particular kind of cosmic ray activity. The energies involved in cosmic rays cover a wide range, from below 1 GeV up to at least 1020 eV. In fact, the highest-energy cosmic rays house a hundred million times more energy that any particles produced in the world’s most powerful particle accelerators.
The bulk of cosmic rays below ∼1018 eV are probably galactic in origin, but ultra-high energy rays remain a mystery and may be accelerated from outside the galaxy. In the past, possible sources studied for the UHE cosmic rays have ranged from giant radio galaxies to gamma ray bursts and blazar jets.
If Benford and Protheroe are right, fossil jets may account for at least some of these cosmic rays. And technology is evolving to the point where the theory can be tested. Currently, most fossil jets are below the sensitivity of radio telescopes. But ongoing observations such as those conducted at the Pierre Auger Cosmic Ray Observatory (on the pampas of western Argentina) should tell us much about UHE cosmic ray energies.
The authors note what Auger data may reveal:
Auger may see clumps of UHECR in the sky, which point back to nothing specially luminous. These will be fossils we didn’t know about – plus some we did. The UHECR will then become a diagnostic of regions storing vast magnetic energies, yet perceptible only through equally energetic particles, bringing word of them across great distances.
Great distances indeed. That such structures could exist without our becoming aware of them until now might once have strained credibility. But we live in a universe whose accelerated expansion was not detected until 1998. A universe whose dark matter and dark energy continue to defy our best efforts at definition. Now we have an analysis with predictive capability that can be tested relatively soon through Auger and ultra-high energy neutrino detectors like ANITA. We may well learn that our understanding of what’s out there will have to be revised yet again.
The paper is Benford and Protheroe, “Fossil AGN jets as ultra high energy particle accelerators,” submitted to Monthly Notices of the Royal Astronomical Society and available online.