By Mateo Francisco, Ars Technique
The Crab Nebula (also called M1 or NGC 1952) is visible through small telescopes, which has allowed astronomers to observe its growth and evolution since the supernovae that created it became visible in 1054 CE. In 1968 a pulsar was found in the center of the Crab. This rapidly spinning neutron star is the core of the star that went supernova to form the nebula. In the intervening decades, X-ray, gamma-ray, and radio observations have mapped the region of the nebula closest to the pulsar. During that mapping, it became clear that the Crab pulsar is one of the brightest sources of gamma rays observable from Earth.
[partner id=”arstechnica”]Despite all those observations, we still don’t fully understand the Crab’s precise gamma-ray spectrum, particularly the pulses of intense gamma radiation recently observed by the Fermi Gamma-ray Space Telescope. Existing models certainly describe well much of the complex interaction between the pulsar’s intense magnetic fields and the outward-flowing winds of charged particles. But no plan alone seems sufficient to cover all the observed phenomena.
A potentially promising new model, proposed by FA Aharonian, SV Bogovalov and D. Khangulyan, may fill in some of these blanks. It proposes that areas near the pulsar act as fast particle accelerators, but do not propel electrons or heavier particles to the same extent.
Pulsars are extremely small despite their large mass: according to typical neutron star models, the Crab pulsar is about 30 kilometers in diameter, but it contains almost twice the mass of our Sun. The intense gravitational influence and Pulsars’ rapid rotation places them firmly in the realm of relativity, while the intense magnetic fields carry the enormous amounts of energy we normally find in particle accelerators.
In the region immediately surrounding the Crab pulsar, there is enough energy to produce electron-positron pairs, which flow into the surrounding gas. This total flow is the pulsar winda plasma (an electrically neutral substance consisting of separate positive and negative charges) that moves very close to the speed of light.
Near the pulsar, most of the pulsar wind energy is in the form of electromagnetic energy from the pulsar itself. Further afield, the energy mainly comes in the form of kinetic energy of fast-moving plasma particles. It is not entirely clear how this transition occurs; In particular, it is difficult to agree with 2011 observations of intensely energetic gamma-ray pulses.
