However, these powerful radio-loud AGNs are too rare in the cosmos to pass the Ding, Globus and Farrar test: They couldn’t possibly be tracers for the large-scale structure. In fact, within our cosmic neighborhood, there are almost none. “They’re nice sources but not in our backyard,” Rieger said.
Less powerful radio-loud AGNs are much more common and could potentially resemble the continuous model. Centaurus A, for instance, the nearest radio-loud AGN, sits right at the Auger Observatory’s most prominent hot spot. (So does a starburst galaxy.)
For a long time Rieger and other specialists seriously struggled to get low-power AGNs to accelerate protons to Oh-My-God-particle levels. But a recent finding has brought them “back in the game,” he said.
Astrophysicists have long known that about 90 percent of all cosmic rays are protons (that is, hydrogen nuclei); another 9 percent are helium nuclei. The rays can be heavier nuclei such as oxygen or even iron, but experts long assumed that these would get ripped apart by the violent processes needed to accelerate ultrahigh-energy cosmic rays.
Then, in surprising findings in the early 2010s, Auger Observatory scientists inferred from the shapes of the air showers that ultrahigh-energy rays are mostly middleweight nuclei, such as carbon, nitrogen and silicon. These nuclei will achieve the same energy as protons while traveling at lower speeds. And that, in turn, makes it easier to imagine how any of the candidate cosmic accelerators might work.
For example, Rieger has identified a mechanism that would allow low-power AGNs to accelerate heavier cosmic rays to ultrahigh energies: A particle could drift from side to side in an AGN’s jet, getting kicked each time it reenters the fastest part of the flow. “In that case they find they can do that with the low-power radio sources,” Rieger said. “Those would be much more in our backyard.”
Another paper explored whether tidal disruption events would naturally produce middleweight nuclei. “The answer is that it could happen if the stars that are disrupted are white dwarfs,” said Cecilia Lunardini, an astrophysicist at Arizona State University who co-authored the paper. “White dwarfs have this sort of composition—carbon, nitrogen.” Of course, TDEs can happen to any “unfortunate star,” Lunardini said. “But there are lots of white dwarfs, so I don’t see this as something very contrived.”
Researchers continue to explore the implications of the highest-energy cosmic rays being on the heavy side. But they can agree that it makes the problem of how to accelerate them easier. “The heavy composition towards higher energy relaxes things much more,” Rieger said.
As the short list of candidate accelerators crystallizes, the search for the right answer will continue to be led by new observations. Everyone is excited for AugerPrime, an upgraded observatory; starting later this year, it will identify the composition of each individual cosmic ray event, rather than estimating the overall composition. That way, researchers can isolate the protons, which deflect the least on their way to Earth, and look back at their arrival directions to identify individual sources. (These sources would presumably produce the heavier nuclei as well.)
Many experts suspect that a mix of sources might contribute to the ultrahigh-energy cosmic-ray spectrum. But they generally expect one source type to dominate, and only one to reach the extreme end of the spectrum. “My money is on that it’s only one,” said Unger.