How black holes change gear

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Black holes are extremely powerful and efficient engines that not only swallow up matter, but also return a lot of energy to the Universe in exchange for the mass they eat. When black holes attract mass they also trigger the release of intense X-ray radiation and power strong jets. But not all black holes do this the same way. This has long baffled astronomers. By studying two active black holes researchers at the SRON Netherlands Institute for Space Research have now gathered evidence that suggests that each black hole can change between two different regimes, like changing gears of an engine. The team’s findings will be published in two papers in the journal Monthly Notices of the Royal Astronomical Society.

Black hole jets – relativistically outflowing, lighthouse-like beams of material – can have a major impact on the evolution of their environment. For example, jets of super-massive black holes can blow huge bubbles in clusters of galaxies warming the gas in the centre. Another stunning example of what black hole jets can do is known as Hanny’s Voorwerp, a cloud of gas where stars started forming after it was hit by the jet-beam of a black hole in a neighbouring galaxy. That is why research into the way black holes produce energy and distribute is important. But until recently, much has remained uncertain. In 2003 it became clear from the astronomical observations that there is a connection between the X-ray emission from a black hole and the jet outflow . This connection needs to be explained if we want to understand how the black hole engine works. In the first years after this connection was discovered, it seemed that it was the same for all feeding black holes, but soon enough oddballs started to be discovered. These unusual examples still follow a relation between the energy released in the X-ray emission and that put in the jet ejection. But the proportion differs from that of the “standard” black holes. As the number of oddballs grew, it seemed that there were two groups of black hole engines working in a slightly different way, as if one were running on petrol and the other on diesel.

Black hole jets 1

Changing gear

Astronomers struggled for years trying to justify this difference based on the properties of the two groups of black holes. However, to no avail. But recently a step forward was made: an international team of astronomers led by Michael Coriat (now University of Southampton) found a black hole that seemed to switch between the two flavours of X-ray/jet connection, depending on its luminosity. This suggested that black holes do not necessarily come with two different engines, but that each black hole can run in two different regimes, like two gears of the same engine.

Artist impression of the two different "gears/modes" in the black hole accretion (P. Jonker/Rob Hynes).

Now Peter Jonker and PhD-student Eva Ratti- researchers from SRON Netherlands Institute for Space Research – have recently taken an important step forward in the attempts to solve this puzzle. Using X-ray observations from the Chandra X-ray Observatory and radio observations from the Expanded Very Large Array in New Mexico they closely observed two black hole systems until the end of one of their recent feeding frenzies. Eva Ratti: “i We found that these two black holes could also ‘change gear’, demonstrating that this is not an exceptional property of one peculiar black hole. This suggests that this behavior might be common among black holes. Moreover, we found that the switch happens at a similar X-ray luminosity for all the three black holes.”

These discoveries, soon to be published in two papers on the Monthly Notices of the Royal Astronomical Society, provide new important input for theoretical models that aim to explain both the functioning of the black hole engine itself and its impact on the black hole environment. The articles are now on line:

The black hole candidate MAXI J1659–152 in and towards quiescence in X-ray and radio
The black hole candidate XTE J1752?223 towards and in quiescence: optical and simultaneous X-ray–radio observations