Observations reveal Crab Nebula's polarised emissions

The polarisation of Crab X-rays reveals how and where they are produced in the extreme environment of the nebula, says Mark Pearce, Professor of Physics at KTH Royal Institute of Technology and lead author of the study.

“Our measurements indicate that the X-rays come from an organised region in the vicinity of the pulsar at the centre of the nebula," Pearce says. "Electrons gyrating around magnetic field lines in this region produce the X-rays".

"The measurements are made in an unexplored energy range, so they provide new information which will help to solve the puzzle of how high energy radiation is generated.”

In 1054 CE, Chinese astronomers recorded the appearance of a new bright star on the sky - an event we now refer to as a supernova, or exploding star. The aftermath of this cataclysmic event was a rapidly rotating neutron star: the Crab pulsar, barely 15 km in diameter but with a mass equal to our solar system's Sun, surrounded by an expanding nebula of particles and radiation.

Neutron stars are a kind of ultra-dense zombie sun that forms when a star exhausts its fuel and collapses upon itself due to the force of its own gravity. If they were any denser, they'd be black holes.

Pearce says that while detecting the Crab’s X-rays is routine business for research satellites, examining the polarisation of these emissions — that is, the plane in which the radiation waves oscillate — is new territory.

“Neutron stars are fascinating objects,” Pearce says. “The Crab pulsar rotates around an axis 30 times per second producing flashes of X-rays – a sort of cosmic lighthouse. The X-rays arise from the acceleration of electrons in intense magnetic fields (10 trillion times stronger than the earth’s magnetic field), up to energies typically a hundred times higher than obtainable at the LHC accelerator.”

The article in Scientific Reports sheds new light on the pulsar through novel measurements conducted by a balloon-borne telescope, PoGO+ (“PoGO plus”), flown at the top of the atmosphere in summer 2016.

Just like visible light or radio waves, X-rays are electromagnetic and they can be polarised, or in other words, the electric field can oscillate in a specific plane. Usually, the polarisation cannot be measured by X-ray telescopes, so researchers miss out on some of the information carried by these X-ray messengers, Pearce says.

The PoGO+ mission was developed specifically to measure the polarisation of X-rays from the Crab and other celestial bodies, with the aim of opening a new observational window on these objects.

Since X-rays are readily absorbed by the earth’s atmosphere, observations need to take place high in the stratosphere. In the early hours of July 12, 2016, an enormous, 1.1 million-cubic-metre helium balloon carrying a specially built telescope was released from the SSC Esrange Space Centre, near Kiruna in northern Sweden, to do just that.

The PoGO measurements are the first-made in the so-called “hard X-ray” band, covering the energy range 20-160 keV, and provide new data for Crab modeling. Results from the PoGO missions are the first from a dedicated X-ray polarimetry mission in more than 40 years.

PoGO+ reveals that a relatively high fraction, 21%, of Crab X-rays are polarised even though observations encompassed both the pulsar and topologically complex nebula.

Pearce says that this indicates the X-rays originate from a compact region with a well-ordered magnetic field. “The angle of the polarisation plane is aligned to the rotation axis of the pulsar, as expected for electrons which generate X-rays through synchrotron processes while trapped in toroidal trajectories around the pulsar,” he says. “By accurately determining the arrival time of X-rays, PoGO+ was able to distinguish between X-rays which originate from the nebula and pulsar.”

The overall emission was found to be dominated by the nebula. Comparing the measured nebula polarisation angle with that measured at optical wavelengths also indicates that the emission site is associated with the torus – a donut shaped luminous structure in the inner part of the nebula.

Pearce says that the lower polarisation angle seen for the pulsar is in line with results at optical wavelengths - an important confirmation that these more straight-forward measurements are a reasonable proxy for X-ray models.The PoGO+ polarisation results are compatible with those obtained in 2013 from the PoGOLite Pathfinder.

The consistency between these results may help to elucidate the cause of sudden increases in the Crab X-ray intensity which were recently observed. Such flares were unexpected for an object which was long considered to be a celestial standard candle for X-rays.

The PoGO+ mission was supported in Sweden by The Swedish National Space Board, The Knut and Alice Wallenberg Foundation and The Swedish Research Council. In Japan, support was provided by Japan Society for Promotion of Science and ISAS/JAXA.