More Hints of Exotic Cosmic-Ray Origin
Observing the constant rain of cosmic rays hitting Earth can provide information on the “magnetic weather” in other parts of the Galaxy. A new high-precision measurement of two cosmic-ray elements, boron and carbon, supports a specific model of the magnetic turbulence that deflects cosmic rays on their journey through the Galaxy. The data, which come from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station, appear to rule out alternative models for cosmic-ray propagation. The failure of these models—which were devised to explain recent observations of cosmic-ray antimatter—implies a possible exotic origin for some cosmic rays.
The majority of cosmic rays are particles or nuclei produced in supernovae or other astrophysical sources. However, as these so-called primary cosmic rays travel through the Galaxy to Earth, they collide with gas atoms in the interstellar medium. The collisions produce a secondary class of cosmic rays with masses and energies that differ from primary cosmic rays. To investigate the relationship of the two classes, astrophysicists often look at the ratio of the number of detections of two nuclei, such as boron and carbon. For the most part, carbon cosmic rays have a primary origin, whereas boron is almost exclusively created in secondary processes. A relatively high boron-to-carbon (B/C) ratio in a certain energy range implies that the relevant cosmic rays are traversing a lot of gas before reaching us. “The B/C ratio tells you how cosmic rays propagate through space,” says AMS principal investigator Samuel Ting of MIT.
Previous measurements of the B/C ratio have had large errors of 15% or more, especially at high energy, mainly because of the brief data collection time available for balloon-based detectors. But the AMS has been operating on the Space Station for five years, and over this time it has collected more than 80 billion cosmic rays. The AMS detectors measure the charges of these cosmic rays, allowing the elements to be identified. The collaboration has detected over ten million carbon and boron nuclei, with energies per nucleon ranging from a few hundred MeV up to a few TeV.
The B/C ratio decreases with energy because higher-energy cosmic rays tend to take a more direct path to us (and therefore experience fewer collisions producing boron). By contrast, lower-energy cosmic rays are diverted more strongly by magnetic fields, so they bounce around like pinballs among magnetic turbulence regions in the Galaxy. Several theories have been proposed to describe the size and spacing of these turbulent regions, and these theories lead to predictions for the energy dependence of the B/C ratio. However, previous B/C observations have not been precise enough to favor one theory over another. The AMS data show very clearly that the B/C ratio is proportional to the energy raised to the -1/3 power. This result matches a prediction based on a theory of magnetohydrodynamics developed in 1941 by the Russian mathematician Andrey Kolmogorov .
These results conflict with models that predict that the B/C ratio should exhibit some more complex energy dependence, such as kinks in the B/C spectrum at specific energies . Theorists proposed these models to explain anomalous observations—by AMS and other experiments—that showed an increase in the number of positrons (anti-electrons) reaching Earth relative to electrons at high energy (see 3 April 2013 Viewpoint). The idea was that these “excess” positrons are—like boron—produced in collisions between cosmic rays and interstellar gas. But such a scenario would require that cosmic rays encounter additional scattering sites, not just magnetically turbulent regions. By ruling out these models, the AMS results support the alternative explanation—a new primary cosmic ray source that emits positrons. Candidates include pulsars and dark matter, but a lot of mystery still surrounds the unexplained positron data.
Igor Moskalenko from Stanford University is very surprised at the close match between the data and the Kolmogorov model. He expected that the ratio would deviate from a single power law in a way that might provide clues to the origin of the excess positrons. “This is a dramatic result that should lead to much better understanding of interstellar magnetohydrodynamic turbulence and propagation of cosmic rays," he says. "On the other hand, it is very much unexpected in that it makes recent discoveries in astrophysics of cosmic rays even more puzzling.”
This research is published in Physical Review Letters.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.
- A. N. Kolmogorov, “The Local Structure of Turbulence in Incompressible Viscous Fluid for Very Large Reynolds Numbers,” Dokl. Akad. Nauk SSSR 30, 301 (1941).
- A. E. Vladimirov, G. Jóhannesson, I. V. Moskalenko, and T. A. Porter, “Testing the Origin of High-Energy Cosmic Rays,” Astrophys. J. 752, 68 (2012).