It’s always nice when something exceeds your expectations. When the thing in question is a spinning neutron star and your expectations are how much energy it releases, though, it’s very puzzling.
Gamma-ray observations of the Crab Nebula, home to a young pulsar, have surprised astronomers. The pulsar emits photons with an energy much larger than what was believed theoretically possible for this type of star.
The Crab Pulsar is a young neutron star with a mass 1.5 times larger than the Sun but a radius of only 10 kilometers (6 miles). It was created by a supernova, which was visible from Earth in 1054. It spins on its axis 30 times per second, and it is surrounded by an intense magnetic field, 10 trillion times stronger than the Sun's. The field creates a region around the star, called a magnetosphere. As the star and the magnetosphere rotate, they create an electric field that rips electrons from the surface and propels them forward into space. The accelerated particles then produce high-energy photons like those detected.
Astronomers used the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) observatory to look at the gamma-rays emitted by the pulsar. They discovered incredibly energetic photons, reaching the teraelectronvolt (TeV) scale, equivalent to the kinetic energy of a flying mosquito.
Emma de Oña Wilhelmi, principal investigator of this observation program, said in a statement: "We performed deep observation of the Crab pulsar with MAGIC to understand this phenomenon, expecting to measure the maximum energy of the pulsating photons."
"The new observations extend this tail to much higher, above TeV energies, that is, several times more energetic than the previous measurement, violating all the theory models believed to be at work in neutron stars," added Roberta Zanin from the University of Barcelona, a co-author on the study.
The light emission from a pulsar can resemble that of a lighthouse – as the pulsar spins so does the magnetic field. By looking at when different types of light reach instruments, it’s possible to establish where the photons have been emitted from. Astronomers found that the very energetic photons arrived to us at the same time as the other types of light, suggesting that they are all emitted fairly close to the pulsar within the magnetosphere.
David Carreto Fidalgo from Complutense University of Madrid, another co-author on the study, said: "How and where this effect is achieved in such a small region challenges our knowledge of physics."