1. Home
  2. News & Publications
  3. Research News

Dec. 15, 2006 Research Highlight Physics / Astronomy

Cosmic rays track Milky Way’s rotation

Elusive sources of high-energy particles come into focus

image of the Tibet Air Shower Array Figure 1: The Tibet Air Shower Array that was built to observe high-energy cosmic rays. © Tibet ASγ Collaboration

Earth is bombarded constantly by cosmic rays, high-energy particles that travel close to the speed of light. The source of the most energetic cosmic rays has long been a mystery, but now astrophysicists have made a breakthrough that may help to identify their origins1.

Each of the high-energy cosmic rays—mostly protons — hit Earth’s atmosphere with the same energy as a speeding baseball. These cosmic rays shatter air molecules into fragments that cascade to the ground in a shower, and can be spotted by detectors such as the Tibet Air Shower Array (known as Tibet ASγ) (Fig. 1).

Low energy cosmic rays are more abundant, but are easily deflected by local magnetic fields, so any sense of where they originated is lost. High energy rays are less likely to be deflected, hence more easily sourced—but they are exceedingly rare.

Still, over the past few years, Tibet ASγ has observed tens of billions of cosmic ray events in the high-energy range, measured in trillions of electron volts. These events are distributed unevenly around the sky, because high-energy cosmic rays are likely to be generated in very specific conditions found only in certain parts of the Milky Way.

Earth’s motion also plays a part. Experiments show that there are more high energy cosmic ray impacts on the ‘leading face’ of Earth as it rotates around the sun, just as the front windshield of a moving car will have more bugs splattered on it. However, scientists have long been unsure whether the rotation of our Solar System around the galactic center also plays a similar part in concentrating the rays in certain areas.

Now, results from Tibet ASγ show that it does not. Instead, the sources of high-energy cosmic rays appear to rotate at the same rate as the galactic magnetic field itself, ruling out any contribution from Earth’s journey around the Milky Way.

Understanding precisely how these cosmic rays move through space should help to pin down their origins, says Harufumi Tsuchiya from RIKEN’s Discovery Research Institute and part of the Tibet ASγ collaboration. It should also reveal details of the magnetic fields around the sun and between the stars that affect the cosmic rays' trajectories, he adds.

One place to start looking is the Cygnus region of our Galaxy. The team found elevated levels of high-energy cosmic rays streaming from that part of space. They hope to investigate the source further in the quest to find the origin of cosmic rays.

References

  • 1. Amenomori, M., Ayabe, S., Bi X.J., Chen, D., Cui, S.W., Danzengluobu, Ding L.K., Ding, X.H., Feng C.F., Zhaoyang Feng, et al. Anisotropy and corotation of Galactic cosmic rays. Science 314, 439–443 (2006). doi: 10.1126/science.1131702

Top