The pulsar at the heart of the famous Crab nebula is bursting with energy. This was just confirmed by the MAGIC collaboration operating two large telescopes on the Canary island La Palma. MAGIC observed the pulsar in the gamma ray range above 50 GeV, where instruments were almost blind so far and found out periodic emission of short pulses stretching till the energies as high as 400 GeV. This is 50-100 times more than expected from theory. Astrophysicists are at a loss to provide a satisfactory explanation.

The observed neutron star in the Crab nebula is one of the best known pulsars. It rotates around its own axis 30 times per second and its magnetic field is 100 million Tesla. This is 1000 billion times stronger than that of the Earth. The pulsar, which is located at a distance of around 6000 light years from Earth in the constellation of Taurus, powers the surrounding Crab nebula. The pulsar and its nebula are the remnants of a supernova that exploded in 1054 AD and, for a while, was visible to the naked eye even in the day sky.

The neutron stars are extremely dense objects with masses similar to that of the Sun but with diameters of only about 10 km. The rotation period of neutron stars is extremely regular and fast; a ‘day’ for them lasts between 1/1000 second and 10 seconds. While rotating, a neutron star continuously generates charged particles, mainly electrons and positrons (positively charged electrons). The particles travel along magnetic field lines, which in turn rotate with the same speed as the neutron star itself. The particles emit beamed radiation over much of the electromagnetic spectrum, from radio waves to gamma rays. Whenever such a beam crosses our line of sight for a short moment, its emission becomes visible to us, much like the signal of a lighthouse seen from a distance.

A few years ago the MAGIC telescope discovered gamma rays from the Crab pulsar at energies above 25 GeV, which was a really big surprise. Scientists concluded that this radiation had to be produced at least 60 km above the neutron star’s surface, because the high energy photons are shielded very effectively by the magnetic field of the star. As a consequence, a source of gamma rays located very close to its surface could not be detected at such high energies, which ruled out one of the main theories of the periodic emission from the Crab pulsar.

About one and a half year ago the MAGIC observations showed that the gamma-ray pulsations are stretching to the higher energy of 100 GeV.

Only half a year ago, the VERITAS collaboration detected pulsed gamma rays with energies even beyond 100 GeV, which again, by far exceeded expectations. Now, after the analysis of collected over the last two years data, MAGIC provides the most detailed and precise measurement of the periodic emission throughout the energy range of 50 - 400 GeV. The duration of pulses turn out to be 1/1000 second short.

The recent measurements of MAGIC, together with those of the orbiting FERMI satellite at much lower energies, provide an uninterrupted spectrum from 0.1 GeV to 400 GeV. These clear observational facts create major difficulties for most of the existing pulsar theories that until recently were predicting significantly lower boundaries for highest energy emission. At the same time these measurements provide pulsar theorists with reliable input for new challenges in attempting to explain the enigmatic pulsars.

A newly developed model explains the phenomenon by a cascade-like process producing secondary particles that potentially overcome the barrier of the pulsar magnetosphere. Another explanation, recently reported in the journal Nature, connects our observations with the equally puzzling physics of the dark pulsar wind – a flow composed of electrons and positrons and electromagnetic radiation which eventually develops into the surrounding nebula.

Still, even though these new models attempt to largely explain both the extreme high energies and the short duration of the pulses, further improvements are necessary for achieving a satisfactory agreement with the observational results. Astrophysicists therefore hope that future observations will help to solve the puzzle. This could shed new light on this class of astrophysical objects and, in particular, on one of the best studied objects in our galaxy: the Crab pulsar and nebula.

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