Immediately, some stocks increased: in particular, three types of candidate objects that thread the needle are relatively common in the cosmos but potentially special enough to produce Oh-My-God particles.
In 2008, Farrar and a co-author offers that cataclysms called tidal disturbance events (TDEs) could be the source of ultra-high energy cosmic rays.
A TDE occurs when a star pulls an Icarus and gets too close to a supermassive black hole. The front of the star feels so much more gravity than its back that the star tears into a thousand pieces and swirls in the abyss. The whirlpool lasts about a year. While this lasts, two jets of matter – the subatomic shreds of the disturbed star – shoot out of the black hole in opposite directions. The shock waves and magnetic fields from these beams could then conspire to accelerate nuclei to ultra-high energies before launching them into space.
Tidal disturbance events occur about once every 100,000 years in every galaxy, which is the cosmological equivalent of happening everywhere and all the time. Since galaxies trace the distribution of matter, TDEs could explain the success of Ding, Globus and Farrar’s continuous model.
Plus, the relatively brief flash of a TDE solves other puzzles. By the time the cosmic ray of a TDE reaches us, the TDE will have been dark for thousands of years. Other cosmic rays of the same TDE can take separate curved paths; some may not arrive for centuries. The transient nature of a TDE could explain why there appear to be so few patterns in the directions of arrival of cosmic rays, without a strong correlation with the positions of known objects. “I tend to believe now that they’re transient, for the most part,” Farrar said of the origins of rays.
The TDE hypothesis got another boost recently, from an observation reported in Nature astronomy in February.
Robert stein, one of the authors of the article, was operating a telescope in California called the Zwicky Transient Factory in October 2019 when an alert arrived from the IceCube neutrino observatory in Antarctica. IceCube had spotted a particularly energetic neutrino. High energy neutrinos are produced when even higher energy cosmic rays scatter light or matter in the environment where they are created. Fortunately, neutrinos, being neutral, travel to us in straight lines, so they point directly at the source of their parent cosmic ray.
Stein rotated the telescope in the direction of arrival of the IceCube neutrino. “We immediately saw that there was a tidal disturbance event from the position from which the neutrino had arrived,” he said.
The match makes it more likely that TDEs are at least one ultra-high energy cosmic ray source. However, the energy of the neutrino was probably too low to prove that TDEs produce the most energetic rays. Some researchers strongly wonder if these transients can accelerate nuclei to the end of the observed energy spectrum; theorists are still exploring how events might accelerate particles in the first place.
Meanwhile, other facts have diverted the attention of some researchers.
Cosmic ray observatories such as Auger and the Telescope Array have also found a few hot spots – small, subtle concentrations in the directions of arrival of higher energy cosmic rays. In 2018, Auger published the results of a comparison of its hot spots with the locations of astrophysical objects a few hundred million light-years away. (Cosmic rays from farther away would lose too much energy in mid-point collisions.)
In the cross-correlation contest, neither type of object performed exceptionally well – which is understandable, given the experience of deflecting cosmic rays. But the strongest correlation surprised many experts: about 10% of the rays came from less than 13 degrees from the directions of so-called “star galaxies”. “They weren’t on my plate originally,” said Michael unger from the Karlsruhe Institute of Technology, member of the Auger team.