Astronomers have made an unprecedented observation of extremely high-energy gamma rays emanating from a pulsar. The intensity of this radiation challenges our current understanding of these peculiar stars and their particle acceleration mechanisms.
Pulsars are rapidly spinning neutron stars with strong magnetic fields. Thousands of pulsars have been discovered, but only four have been detected emitting gamma rays of sufficient power to be observed by ground-based telescopes. Among these four, only one, known as the Vela pulsar, emits gamma rays composed of photons with energies exceeding one teraelectronvolt (TeV) – roughly equivalent to the kinetic energy of a flying mosquito, which is quite significant for a fundamental particle.
Arache Djannati-Ataï at Paris Cité University in France and his colleagues conducted observations of the Vela pulsar using the High Energy Stereoscopic System, an array of telescopes in Namibia. They discovered that the Vela pulsar emits gamma rays with energies surpassing 20 TeV, which is approximately 20 times more energetic than the highest-energy radiation detected from any other pulsar.
Current explanations of particle acceleration in pulsars fall short in accounting for this finding. Additionally, the method we use to detect gamma rays, by observing the particles produced when they interact with Earth’s atmosphere, implies that they may possess even higher energies than the measured 20 TeV. However, the lack of sufficient data prevents us from confirming this possibility. “We are establishing a lower limit on the energies of these high-energy particles, and this is already challenging for existing models,” says Djannati-Ataï. “But even if we go beyond 20 TeV, for now, we have no evidence of a cutoff energy.”
Two competing models explain the acceleration of photons by pulsars, both involving high-energy electrons colliding with lower-energy photons that constitute gamma rays. The discrepancy lies in how these electrons are accelerated in the first place – either by being propelled away from the pulsar due to interactions with its magnetic field or by being rapidly propelled as they follow the pulsar’s swift rotation. Nevertheless, neither of these models can reconcile the discovery of gamma rays with seemingly no upper limit to their energy. Djannati-Ataï remarks, “How and where the electrons are accelerated – that is the question.” Until we resolve this, our understanding of pulsars, their impact on their surroundings, and the reasons behind the Vela pulsar’s remarkably intense radiation will remain incomplete.
