Shedding new light on black hole ejections

3 March 2020

A research group led by Oxford’s Department of Physics has observed a black hole ejecting material at close to the speed of light, out to some of the largest separations ever seen. These observations have allowed a deeper understanding into how black holes feed into their environment.

The group focuses on studying transient astrophysical systems – things that change brightness on short timescales. The system studied in this instance contains a dynamically confirmed black hole within our galaxy and another star (not too dissimilar from our sun) orbiting one another. The black hole, due to its strong gravitational pull, syphons material from its companion star and accretes it.

Extensive observing campaign

Joe Bright, DPhil student at Oxford University’s Department of Physics and lead author of the study explains: ‘Most importantly to this work is the fact that the material is not all lost into the black hole. Outflows are launched away from the black hole at extreme velocities – almost the speed of light – and can be observed with radio telescopes.

‘Our group in Oxford, along with international collaborators, lead an extensive observing campaign on this particular system, known as MAXI J1820+070, after it went into outburst in the summer of 2018. This in itself was remarkable as this type of transient astrophysical system mostly accretes a very small amount of material and so can’t be seen; they do however occasionally go into outburst and only then are they observable.

‘Our campaign included telescopes in the UK, America and the newly operational MeerKAT telescope in South Africa. With these facilities we were able to track the connection between accretion and outflows. More excitingly we were able to observe the system launching ejections of material, and to track these ejections over a wide range of separations from the black hole.’

Successfully tracking ejections

The group successfully continuously tracked these ejections to extreme distances from the black hole with a range of radio telescopes and the final angular separation is among the largest seen from such systems. The ejections are moving so fast that they appear to be moving faster than the speed of light – they are not, rather this is a phenomenon known as apparent superluminal motion.

Co-lead of the project, and author on the paper, Rob Fender said "We've been studying these kind of jets for over 20 years and never have we tracked them so beautifully over such a large distance. To see them so early on in the operation of a new facility like MeerKAT is fantastic, and - as is often the case - teaches us not to confidently predict what we're going to see in the future".

Joe concludes: ‘Using our radio observations we were able to better estimate how much energy is contained in these ejections using a novel method for this type of system. Galactic black holes, such as MAXI J1820+070, are thought to be miniature versions of the supermassive black holes that are found at the centre of galaxies. The feedback from these black holes is thought to be a vital component regulating the growth of galaxies – but these systems evolve on timescales much longer than a human lifetime. Their galactic counterparts, however, evolve quickly and are therefore the perfect systems to study the feedback process and its connection to accretion.’

An extremely powerful long-lived superluminal ejection from the black hole MAXI J1820+070, Nature Astronomy, 2 March 2020


Physics in focus

What is accretion?
Accretion is a process in which material falls onto an object – in this case a black hole – through a gravitational interaction. Accreted material loses its gravitational potential energy which is converted to radiation (primarily optical and X-ray light) in an accretion disk – a donut like structure orbiting the black hole. When a black hole is involved, accretion is one of the most efficient energy conversion processes known to exist.

Image © South African Radio Astronomy Observatory (SARAO)