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Waves of charged particles slamming into gas and dust may be responsible for unexpected super-bright flashes in the Crab nebula.

The Crab, located about 6500 light years away in the constellation Taurus, is the remains of a star that exploded as a supernova in 1054 AD. At its heart is a compact neutron star – an ultra-dense object the size of a city but with several times the mass of the sun.

That star is a pulsar, meaning its magnetic field generates intense beams that, as the star spins on its axis, shine in a similar way to the beam from a lighthouse. The Crab nebula is the most powerful pulsar known in the Milky Way.

Pulsars are some of the most regular objects in the cosmos; the Crab nebula’s spins around 33 times per second and produces steady electromagnetic radiation. This is why astronomers use them to calibrate their instruments.

But a wrinkle appeared in 2011, when two telescopes observed unusual short-lived gamma ray bursts in the Crab. These exceeded 100 megaelectronvolts, hundreds of times brighter than the nebula’s normal emissions.

Pockets that spark

The source of the flashes was a mystery. Some suspected it had to do with the pulsar’s magnetic fields splitting or becoming knotted as they move away from the star and into surrounding material.

Now, new research pins the gamma ray emissions on charged particles like electrons and positrons that flow from the neutron star at near light speed, like a constant breeze.

“We propose that at some moments you have pockets inside this wind” where the density of electrons and positrons drops, says at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.

Lines of electromagnetic force usually maintain a constant current. When they encounter these lower density pockets where the charged particles contain more energy per particle than in higher density regions, that current is disrupted. Some of the energy in the particles gets converted into kinetic energy, causing the electrons and positrons to accelerate and smash into the nebula’s gas and dust.

The effect is similar to the behaviour of a circuit containing an inductor. Should the current in the circuit suddenly drop, the inductor – which acts to smooth out the electric current – will try to rapidly counteract the loss with a huge pulse, often resulting in a spark.

Polarised particles

Giacinti and his co-author , also at the Max Plank Institute, calculated that this inductive sparking phenomenon should produce bursts in the Crab nebula with almost exactly the same energy as the observed emissions.

They propose checking the idea by looking for polarisation in the gamma ray flares. When accelerated, the electrons and positrons should give off what’s known as synchrotron radiation, which is always polarised – in this case gamma rays. They also suggest looking for similar outbursts in the 100 or so other known nebulae containing pulsar winds.

Their proposal has stood out for its novelty. “I myself have never really seen that before,” says at the University of California, Los Angeles. But determining if it is indeed the mechanism behind the Crab’s gamma ray flares will probably require more sophisticated modelling and a great deal of time, he says.

Read more: Fuzzy pulsars orbiting black holes could unmask quantum gravity