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Astronomy

Shooting down jet theories with an impossible discovery

→ Reference J. van den Eijnden, N. Degenaar, T.D. Russell, R. Wijnands, J.C.A. MillerJones, G.R. Sivakoff and J.V. Hernández Santisteban, Nature 562 (7726), 233–235 (2018). doi:10.1038/s41586018-0524-1

→ Ask any astronomer a question that they cannot answer, and they’ll likely claim it is somehow related to magnetic fields. The effect of magnetism in space – the rug astronomers thankfully sweep their unsolved problems under – is notoriously hard to understand. That holds especially in studying the enormous and powerful streams of matter called ’relativistic jets’. With recent discoveries, we are trying to clarify the link between magnetic fields and jets by, paradoxically, realising that we understand them less than before.

“A jet contains ionised particles, such as protons, electrons and positrons travelling close to the speed of light”

Shooting from the (neutron) stars Massive stars end their lives with a bang: as their nuclear fuel is exhausted, they explode in a supernova, expelling most of their gas, but leaving a tiny, collapsing core behind. Depending on the star’s mass, this core can have two fates: while the heaviest stars see their centres collapse into a black hole, lighter stars leave behind an extremely compact core. In this core, more than the mass of the Sun is compressed to the size of Amsterdam, reaching densities exceeding that of atomic nuclei. Although we do not know what matter looks

JAKOB VAN DEN EIJNDEN is PhD student at the Anton Pannekoek Institute for Astronomy, UvA.

like in those circumstances, we call these exotic stellar remnants neutron stars. Most stars don’t live in isolation, however, but in binary systems with another star. When one of them turns into a neutron star, billions of years of peaceful coexistence can be disturbed; since an object’s gravitational sphere of influence grows with its compactness, the neutron star can suddenly capture the outer layers of its neighbour. As these outer layers are stolen, or accreted, conservation of angular momentum (the physical law that keeps a spinning top spinning) forces the gas of the neighbouring star into a disk, forming essentially a giant whirlpool with the neutron star at the sink. But this is not a whirlpool like you have ever seen. Not only can it be millions of kilometres across, from its centre it can spew out an enormous stream of gas, directly into its surrounding interstellar space. A so-called jet, of which an example is shown in Figure 1, contains ionised particles, such as protons, electrons and positrons travelling close to the speed of light. As it travels away from the binary system, the jet ploughs its way through surrounding low-density media, blowing bubbles and heating it up. In the binary itself, the jet removes both a large amount of energy and a significant fraction of the matter plunging towards the neutron star. Magnetic springs Jets are not only launched by neutron stars but are seen everywhere in the Universe, as long as accretion occurs. We observe them both when we look at forming stars and ← Figure 1 Maybe the most famous jet, shot out from the super-massive black hole in the galaxy M87 visible in the top left corner. The size of the jet is over one billion billion km. Credit: NASA.

at the most massive black holes (as in Figure 1), which can weigh as much as a billion Suns. But that does not mean we understand them very well. Their most common explanation involves the magnetism that is so hard to understand in extreme circumstances: whatever the type of object – a black hole for instance – that is accreting, the attracted gas will become magnetised. As this gas rotates in the whirlpool, its magnetic fields rotate as well. They are tangled up, like cotton candy, into a magnetic spring that can launch away material (Figure 2). When we add a neutron star to the mix, everything becomes more complicated: the neutron stars themselves can have incredibly strong magnetic fields, easily millions of times stronger than can be created on Earth. For the most extreme magnetic neutron stars, these fields are too strong to be tangled up into a spring. In other words, in those cases, our most common models for jet launching break down. So, while many basic questions about jets are unanswered – how they are accelerated to their relativistic speeds, or why they are so narrow – jet models make one very clear and testable prediction: strong-magnetic-field neutron stars should not launch a jet. Hunting for the impossible For four decades, this clear prediction was confirmed over and over again. Initially, astronomers turned their telescopes to the most magnetic neutron stars and found no signatures of any jets. Later on, when they instead looked at neutron stars with a ten-thousand times weaker magnetic field, they did detect radio emission: a

Profile for Amsterdam Science

Amsterdam Science magazine issue 9  

Amsterdam Science gives Master’s students, PhD and postdoc researchers as well as staff a platform for communicating their latest and most i...

Amsterdam Science magazine issue 9  

Amsterdam Science gives Master’s students, PhD and postdoc researchers as well as staff a platform for communicating their latest and most i...

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