Astronomers have spotted extremely high-energy gamma rays coming from a pulsar. The extraordinarily high energy of this radiation challenges our models of these strange stars and how they accelerate particles.
Pulsars are rapidly rotating neutron stars with powerful magnetic fields. Astronomers have discovered thousands of them, but only four are known to emit gamma rays powerful enough to be detected by ground-based telescopes. Of those four, only one emits gamma rays made up of photons with energies higher than one teraelectronvolt (TeV) – about the kinetic energy of a flying mosquito, which is quite a lot for a fundamental particle. That one is called the Vela pulsar.
Arache Djannati-Ataï at Paris Cité University in France and his colleagues found that the Vela pulsar is even more unusual than we thought. They observed it with an array of telescopes in Namibia called the High Energy Stereoscopic System, and they found that it emits gamma rays with energies in excess of 20 TeV, about 20 times more energetic than the highest-energy radiation spotted from any other pulsar.
Our current explanations of how pulsars accelerate particles can’t quite account for this. Plus, the way we detect gamma rays through other particles produced in interactions with Earth’s atmosphere means that they could have even higher energies than 20 TeV – we simply don’t have enough data to know for sure. “We are putting a lower limit on the energies of the highest-energy particles, and this is already challenging for existing models,” says Djannati-Ataï. “But even if we go beyond 20 TeV, for the moment we have no indication of a cutoff energy.”
There are two competing models of photon acceleration by pulsars, both involving high-energy electrons slamming into the lower-energy photons that make up gamma rays. The difference comes down to how those electrons are accelerated in the first place: either by being flung away from the pulsar due to interactions with its magnetic field or whipped around to high speeds as they follow the pulsar’s rapid rotation. But each of those models is difficult to reconcile with these newfound gamma rays with no apparent upper limit to their energy. “How and where the electrons are accelerated – that is the question,” says Djannati-Ataï. Until we figure that out, we cannot fully understand pulsars, how they affect their surroundings or why the Vela pulsar is producing such unusually intense radiation.