At the University of Oxford, a contribution has been made by scientists to a significant international study that has captured a unique and captivating space phenomenon.
The study has been reported in the journal Nature.
Researchers have long been interested in X-Ray binary star systems, where two stars orbit around each other with one being either a black hole or a neutron star. Both neutron stars and black holes are made in supernova explosions and are highly dense, thereby giving them a huge gravitational pull.
This gives them the potential to capture the outer layers of the normal star that orbits near it in the binary system, observed as a rotating disc of matter (like a whirlpool) near the black hole or neutron star.
As per the theoretical calculations, such rotating discs must display a dynamic instability: around once an hour, the inner parts of the disc quickly fall onto the black hole or neutron star, following which such inner regions will re-fill, and the process repeats.
So far, this harsh and extreme process has only been noted once directly in a black hole binary system. Initially, it was seen in a neutron star binary system called Swift J1858.6-0814. This breakthrough illustrates that this instability is a common property of such discs (and not caused by the existence of a black hole).
The phenomenon was captured by integrating the data from five ground-based and space-based telescopes, collectively encompassing several wavelengths. The scientific research group, an international collaboration of astronomers headed by the Instituto de Astrofísica de Canarias, developed when the neutron star system was initially discovered in 2018.
Such telescopes include the Karl G. Jansky Very Large Array - one of the world’s most sensitive radio telescopes - situated in New Mexico, composed of 27 massive (25-m diameter) telescope dishes.
Our observations of the radio wavelength data highlighted an important property of these instabilities. We found that when the whirlpool empties, some of the gas is shot into space in so-called ‘radio jets’: narrow beams of gas shot out at speeds close to the speed of light.
Dr Jakob van den Eijnden, Department of Physics, University of Oxford
van den Eijnden led the analysis of the data from the Karl G. Jansky Very Large Array.
The brightness of such jets is noted to be changeable, which is described by blobs of jet material that are being launched at such extreme speeds whenever the disc begins or completes its emptying out (causing peaks in brightness).
When the disc stabilizes, the jets cease, and the brightness decreases. Coming to this conclusion was only possible by making a comparison of the variability noted with telescopes throughout the electromagnetic spectrum—from radio to X-Ray wavelengths—which concurrently probes the disc and the jet’s behavior.
I think that the international collaboration and involvement of many early-career researchers is one of the most exciting aspects of this work. We analyzed a truly unique dataset, that was extremely challenging to collect, because the gas capturing process is ‘transient’: it takes place for only a couple of months, unpredictably, before shutting off again.
Dr. Jakob van den Eijnden, Department of Physics, University of Oxford
Dr. van den Eijnden added: “This discovery, only the second example of these instabilities, also highlights the rarity of this behavior. Therefore, finding more examples across different types of binary systems is a first priority. Due to the transient nature of this process, it is unpredictable when we will get another chance. By then, we will need to be prepared to repeat our international observing efforts.”
Vincentelli, F. M., et al. (2023) A shared accretion instability for black holes and neutron stars. Nature. doi.org/10.1038/s41586-022-05648-3.