### Origin of Ultra-High-Energy Cosmic Rays (UHECRs)
Ultra-high-energy cosmic rays (UHECRs) are extremely energetic subatomic particles arriving from space. Scientists debate their origins.
Hypothesis 1
UHECRs originate within the Milky Way galaxy. They are accelerated by the intense magnetic fields of rapidly spinning neutron stars (magnetars) located in supernova remnants. Because they are produced locally, these particles can reach Earth with extremely high energies. Furthermore, their arrival directions should show a statistically significant clustering along the galactic plane (the disk of the Milky Way).
Hypothesis 2
UHECRs originate from extragalactic sources, specifically Active Galactic Nuclei (AGNs)—supermassive black holes at the centers of distant galaxies. These particles are accelerated by powerful relativistic jets. Because AGNs are distributed throughout the universe, the arrival directions of UHECRs should be isotropic (distributed evenly across the sky) or correlate with the positions of nearby galaxies outside the Milky Way, rather than the galactic plane.
A newly detected group of UHECRs with energies exceeding was found to have arrival directions that do not cluster along the galactic plane, but instead correlate closely with the positions of a cluster of nearby AGNs located approximately million light-years away. Which of the following statements best describes how this new evidence affects the two hypotheses?
- It supports Hypothesis 2 and weakens Hypothesis 1 because the arrival directions correlate with extragalactic objects rather than the galactic plane.Answer
- BIt supports Hypothesis 1 and weakens Hypothesis 2 because the cosmic rays correlate with sources that are relatively close to Earth.
- CIt weakens both Hypothesis 1 and Hypothesis 2 because the detected cosmic rays do not cluster along the galactic plane and are located at a distance of 150 million light-years.
- DIt has no effect on either hypothesis because the detection of cosmic rays does not provide direct physical evidence of their acceleration mechanisms.