In a groundbreaking study published today in Nature Astronomy, researchers have harnessed the unique characteristics of the dense star cluster Terzan 5 to shed new light on the behavior of cosmic rays and the magnetic fields that influence their paths. This celestial laboratory, located in a region of the galaxy currently racing through space, has provided an unprecedented opportunity to measure how cosmic rays alter their trajectories due to fluctuations in interstellar magnetic fields.
Origin of Cosmic rays
Cosmic rays, high-energy particles that travel through space at nearly the speed of light, have long intrigued scientists since their discovery by Austrian-American physicist Victor Hess in 1912. Hess’s observations revealed that radiation levels increased with altitude, even during solar eclipses, indicating that these rays originated from beyond Earth’s atmosphere. This revelation opened a new chapter in understanding radiation sources, distinguishing cosmic rays from the radioactive emissions detected on Earth.
Despite their discovery over a century ago, the exact origins and behaviors of cosmic rays remain partially enigmatic. These particles, which include atomic nuclei and elementary particles like protons and electrons, are subject to deflection by magnetic fields. This phenomenon makes it challenging to trace their origins, as their paths become erratic when they encounter these fields, similar to the way old cathode ray tube (CRT) monitors used magnetic fields to steer electrons.
The vast interstellar magnetic fields continuously fluctuate, causing cosmic rays to scatter in various directions. As a result, these rays do not travel directly from their sources to Earth, but rather spread out, creating a nearly uniform distribution of cosmic rays across the sky. While this general understanding has been established, the specifics of how rapidly these particles change direction due to magnetic fluctuations have remained elusive—until now.
Terzan 5
Terzan 5, a globular cluster of stars near the galactic center, has proven instrumental in advancing our knowledge of cosmic rays. This cluster, which contains a large number of millisecond pulsars—highly magnetized, rapidly rotating neutron stars—accelerates cosmic rays to extreme velocities. Though these cosmic rays do not directly reach Earth due to magnetic deflections, their presence is inferred from gamma rays produced when cosmic rays collide with starlight photons. Unlike cosmic rays, gamma rays are not affected by magnetic fields and travel in straight lines to Earth.
An intriguing aspect of Terzan 5 is the observed displacement of gamma rays from the expected positions of the cluster’s stars. Discovered in 2011, this displacement puzzled astronomers until a novel explanation emerged. Terzan 5 is currently in a rapid, wide orbit that periodically carries it far from the galactic plane. As the cluster plunges through the galaxy at hundreds of kilometers per second, it creates a magnetic “tail,” akin to a comet’s tail in the solar wind.
Terzam 5’s journey towards Earth
Cosmic rays emitted by Terzan 5 initially travel along this magnetic tail. Since the tail is not directed toward Earth, the gamma rays produced by these cosmic rays are beaming away from our line of sight. However, due to magnetic fluctuations, the trajectories of these cosmic rays eventually shift, and some begin to point towards Earth. This process, which takes approximately 30 years, causes the gamma rays to appear displaced from the cluster itself, as they originate from a region about 30 light-years away.
This discovery has allowed scientists to measure, for the first time, the time it takes for magnetic fluctuations to alter cosmic ray directions. This measurement is crucial for testing theories about interstellar magnetic fields and their fluctuations, bringing researchers closer to understanding the cosmic radiation first detected over a century ago by Victor Hess. Through the lens of Terzan 5, astrophysicists have gained valuable insights into the dynamic interactions between cosmic rays and the magnetic fields of our galaxy, marking a significant advancement in the field of astrophysics.
