Astronomers have made a remarkable discovery of extremely high-energy gamma rays coming from a pulsar, challenging current models of these celestial objects and their ability to accelerate particles.
Pulsars are fast-spinning neutron stars with strong magnetic fields. While thousands of pulsars have been discovered, only four are known to emit gamma rays of sufficient energy to be detected by ground-based telescopes. Among them, the Vela pulsar stands out as the only one emitting gamma rays with energies higher than one teraelectronvolt (TeV), equivalent to the kinetic energy of a flying mosquito. This makes the Vela pulsar an intriguing subject for study.
In a recent observation conducted with the High Energy Stereoscopic System, an array of telescopes in Namibia, Arache Djannati-Ataï from Paris Cité University and his colleagues found that the Vela pulsar emits gamma rays with energies exceeding 20 TeV, making it about 20 times more energetic than any other pulsar previously observed.
This discovery challenges the current understanding of how pulsars accelerate particles, as existing models cannot fully explain such high-energy radiation. Furthermore, the scarcity of high-energy gamma rays detected from other sources suggests the possibility of even higher-energy gamma rays, although more data is needed to confirm this. Djannati-Ataï explains, “We are putting a lower limit on the energies of the highest-energy particles, and this is already challenging for existing models. But even if we go beyond 20 TeV, for the moment we have no indication of a cutoff energy.”
There are two competing models for explaining photon acceleration in pulsars, both involving high-energy electrons colliding with lower-energy photons that constitute gamma rays. The discrepancy lies in how these electrons are accelerated initially: either by being expelled from the pulsar through interactions with its magnetic field or by being accelerated to high speeds as they follow the pulsar’s rapid rotation. However, neither of these models fully accounts for the observed gamma rays with no discernible upper limit in energy. Djannati-Ataï states, “How and where the electrons are accelerated – that is the question.” Until this is understood, the nature of pulsars, their impact on the surrounding environment, and the reasons behind the Vela pulsar’s extraordinary radiation remain a mystery.
Article amended on 25 October 2023
We have corrected the reason for uncertainty in the highest gamma ray energies
