Jets are collimated streams of gas and energy that are produced by a variety of astrophysical objects and phenomena. Jets are, for instance, produced by young forming stars, by neutron stars and black holes in X-ray binaries, and by supermassive black holes that lurk in the centers of galaxies. Moreover, jets are seen during explosive, cataclysmic phenomena such as supernova explosions, gamma-ray bursts and bursts of gravitational waves produced by the mergers of compact stars. Despite that jets are so omnipresent in the universe, it is not understood yet how jets are launched in different circumstances and how fast they travel through space.
In an attempt to break new grounds in our understanding of jets, we designed an out-of-the-box experiment to test if thermonuclear explosions that regularly occur on the surface of neutron stars could cause measurable variations in their jets. Our thought was that the radiative power of such explosions would blow away the launch region of jets, causing them to temporarily break down and rebuild thereafter. Observing that would provide unique new insight into how neutron stars produce jets. Since it was not possible to predict how large any effect would be, hence if we would be able to detect anything at all, it was not feasible to obtain observing time for this experiment through regular routes of proposing our idea to a time-allocation committee. Therefore, we used a special opportunity provided by the Dutch national research council (NWO) to apply for a small grant to fund high risk research (the NWO XS grant). With this grant we bought 80 hours of observing time on the Australian Telescope Compact Array (ATCA) radio telescope to perform our crazy jet experiment.
To complement the purchased radio observations that can detect a jet, we obtained time on the INTEGRAL satellite from the European Space Agency (ESA) to detect thermonuclear bursts. Setting up these strictly simultaneous observations was quite a challenge, but we managed to do two runs (each lasting 3 days) on two different bursting neutron stars: 4U 1728-34 and 4U 1636-536. During both runs we detected many bursts and…. we did see a clear jet response!! But it was completely opposite of what we expected: instead of seeing the jets fade (from breaking down) in response to the explosions, we observed a marked brightening of the jet. The fact that the jets persist despite of the bursts provides key information on these outflows are launched and crucial constraints for computer simulations that model the launch of jets.
Another exciting implication from our successful experiment is that the timescale of the response of the jet allowed us to measure, for the first time, the speed of the jet from a neutron star. We found that it is blasted into space at a dazzling speed of 300 million kilometers per hour (or traveling about 90 thousand kilometers in just a single second!). While neutron star jets thus have an enormous velocity, it a factor of 2-3 slower than the velocity measurements that we have for a handful of black holes. This suggest that the properties of the jet-launching object (e.g. their mass, their rotation rate or their magnetic field strength) must play a role in how jets are launched and powered. The important breakthrough of this discovery has opened up a completely new window to understand how jets are connected to the individual properties of a system, which provides us with fundamental insight into the launching of jets on all physical scales.
Never before were we able to anticipate and directly watch how a certain amount of gas got channeled into a jet and accelerated into space. Only the explosions on the surface of the neutron star could give us the clean and isolated view of this process to perform these measurements. Because of the high scientific impact, our results will be published in the journal Nature. Moreover, with the successful demonstration of the experiment it will no longer be difficult to obtain observing time through regular routes to take the same measurements for (many) other neutron stars. Lastly, the spectacular results of our crazy jet experiment demonstrate how valuable it is for science that there are opportunities to support high-risk research. Such projects may, by their very nature, often fail, but it is exciting and fun to try and can turn out to be very high gain.
Russell, Degenaar, van den Eijnden, Maccarone, Tetarenko, Sanchez-Fernandez, Miller-Jones, Kuulkers, Del Santo 2024, Nature 627, 763: Thermonuclear explosions on neutron stars reveal the speed of their jets
Animation: mp4 (source ESA)
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