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Astronomy

clear sign of the presence of a jet. This proved both a beautiful confirmation of the theoretical prediction, and remarkable success in understanding the complicated role of magnetism in these astronomical systems. There is, however, one essential caveat to this match between observations and theory: it relies fundamentally on the fact that we do not see any jets from certain neutron stars. But not seeing something does not guarantee that this something does not exist. It might simply be hard to detect or look different than you expect. For instance, the fact that we have not found any extra-terrestrial life does not necessarily mean that we are alone. Nor does it mean we should stop searching. So, while remarkable, the agreement between observations and models of neutron star jets could be undone by a single observation of a single jet that should not be. Such an ‘impossible’ jet is exactly what our research group in Amsterdam discovered. In the Fall of 2017, we observed the not-so-poetically-named accreting neutron star Swift J0243.6+6124, using one of the world’s most sensitive radio telescopes. This neutron star has an incredibly strong magnetic field, which in theory meant it should not launch any jets. However, to our surprise, a jet is exactly what we saw! This unexpected jet was launched when the neutron star was feeding on as much gas from its companion as it possibly could, providing the fuel needed to power the stream of gas. But despite the enormous appetite of the neutron star, the jet was hundreds of times fainter than others we had seen before. A paradigm shifted What does this discovery mean? First of all, it means that our most successful model for the creation of jets does not work for neutron stars – or at least, not when the neutron star is extremely magnetised. It might still work very well in other systems, such as black holes, forming stars or neutron star with puny magnetic fields, of course. Secondly, it also means that we understand less of the effects of magnetism in these extreme sys-

tems than we previously thought. I would argue that having a more complete view of the complicated effects of magnetism in jet formation is a fundamental step towards a better understanding. Why did it take us until 2017 to make our discovery? Beyond the fact that we even observed this jet, one other aspect of our discovery was particularly puzzling: the jet we observed was, compared to the reservoir of gas attracted by the neutron star, orders of magnitude too faint. This faintness naturally explains why no one saw these ‘impossible’ jets before, so that an incorrect paradigm could emerge: only the most sensitive current-day radio telescopes can detect such weak radiation. In other words, the astronomers that searched for these jets four decades ago, did not stand a chance. But now, we can try to find many more. More questions posed than answered Why was the jet so faint? Being successfully trained as astronomers, we suggested that, maybe, the magnetic field was the culprit again: instead of preventing the jet from forming, maybe it somehow only made it weaker. At the same time, we also kept pointing the radio telescope at this jet, trying to track its evolution. We saw it disappear gradually, as the disk of accreted gas emptied. Then, when a new batch of gas provided new fuel, we were surprised once more: the jet re-formed, as one might expect if more gas is accreted, but did so

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almost instantaneously: as if someone flipped a switch, in only a few days, the jet changed from too weak to detect to as strong as when we first discovered it, despite a much smaller amount of available material than before. Independent of the size of the accreted gas reservoir, our ‘impossible’ jet shows the same brightness – or more accurately phrased, faintness. That suggests that the magnetic strength of the neutron star does not simply regulate the power of the jet as we naively but instinctively suggested after our first discovery. For now, the faintness remains an open question. In fact, many open questions remain. Most importantly, our theories cannot explain that this jet exists at all, and therefore new models have to be designed. Our group, however, consists of observers, so we stick to what we do best. Since we cannot build a neutron star or a jet in a laboratory on Earth, we turn to space’s natural laboratories for our answers. We are trying to find more ‘impossible’ jets, to categorise their properties, and to track their evolution and changes in unprecedented detail. But for now, we can only conclude with: to be continued…  Ω

↑ Figure 2 Artist’s impression of the most common jet model: magnetic fields are spun up like cotton candy, confining and launching gas away from the star or black hole attracting it. Credit: NRAO.

“Our theories cannot explain that this jet exists at all” ↑ Footnote 1 Only’ a few hundred times the largest magnetic field ever created on Earth, instead of more than a million times. → Figure 3 Artist’s impression of Swift J0243.6+6124 and its impossible jets: the neutron star’s magnetic field prevent the rotating gas from getting close to the centre, but somehow a jet is launched nonetheless. Credit: ICRAR/UvA.

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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