Pulsars are known for their regularity and stability. These fast-rotating neutron stars emit radio waves with such consistent pulses that astronomers can use them as a kind of cosmic clock. But recently a pulsar emitted gamma rays with tremendous energy. The gamma rays were the most energetic photons ever observed, with energies of more than 20 teraelectronvolts, and astronomers are struggling to understand how that’s possible. The results were published in Nature Astronomy, which describes the burst of gamma rays emanating from the Vela Pulsar.
The Vela gamma rays were detected by the High Energy Stereoscopic System (HESS). High-energy gamma rays have been seen in pulsars before so that part isn’t surprising. Neutron stars have tremendously strong magnetic fields, and when charged particles are caught in those fields they can be accelerated to a large fraction of the speed of light, which causes them to emit light. The magnetic fields are strongest at the magnetic poles of the neutron star, which is why they often emit powerful beams of radio light. When these beams, or light cones sweep past our direction due to the neutron star’s rotation, we see the regular pulses of light we call pulsars.
Scenarios where accelerated particles might create high-energy gamma rays. Credit: The HESS CollaborationBut in this case, the gamma rays are more intense than the magnetic fields of neutron stars should produce. Vela’s magnetic field is intense, but that on its own can’t explain why these bursts of gamma rays are so powerful. However, the team has noticed that the energetic light cone of the Vela Pulsar is unusually wide. This could be a clue to how it generates such high-energy particles. One idea is that charged particles are being accelerated in a much wider circle initially, and as the magnetic field draws them into the light cone they are already energized. Another is that a combination of the strong magnetic fields and a bulk flow of stellar wind hyper-accelerates the particles.
It will take more research to pin down the answer. But this discovery shows that the interaction of intense magnetic fields and charged particles can occur in unexpected ways, and the upper limits of energy are not bound by our traditional models, This has implications for other powerful magnetic fields, such as those in the vicinity of black holes.
Reference: The HESS Collaboration. “Discovery of a radiation component from the Vela pulsar reaching 20 teraelectronvolts.” Nature Astronomy (2023)
