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Calculating the shadow of a black hole

For the first picture of a black hole, a lot of calculations and models were needed. The astrophysicists of the Radboud University had a leading role in the preparations. What were their contributions to the image of the massive black hole in galaxy M87? And what will the future bring?

Black hole Team Radboud University

"Black hole" team Radboud University. From left to right: Sara Issaoun, Daan van Rossum, Freek Roelofs, Michael Janssen, Christiaan Brinkerink, Heino Falcke, Raquel Fraga-Encinas, Jordy Davelaar, Monica Moscibrodzca.

The moment was finally there in April 2017: the eight telescopes of Event Horizon Telescope (EHT) all pointed at the same point in the sky for the first sighting of a black hole. At almost all of the telescopes an astrophysicist of Radboud University was present. This is remarkable, because almost all of them are theorists and model makers that normally sit behind a desk. Nevertheless, it is important to be present on-site, according to head researcher prof. Heino Falcke. “You need to understand how the telescope works and where your data is coming from.”

After a week of measuring, the EHT astronomers had so much data that they needed to send boxes full of hard drives: sending it digitally would take years. Once the measurements were all in the same computer network, the real work could begin. Radio telescopes do not give a direct image like a telephoto lens, but tell you how many radio waves they observed from the direction on a certain moment.

The signals of all telescopes had to be compared and combined according to the VLBI technique, in order to extract an image of the shadow of the black hole in M87 from this heap of astronomical data. Event Horizon Telescope spent countless hours on this, with leading roles for the astrophysicist from Radboud University.

A crucial step in the data processing is the calibration, in which the brightness of the observed object is compared to known radio sources. Not exactly a glamorous job, but essential if you want clear measurements and not just pretty pictures. And traditionally, it is the domain of a couple of wizards that know how the data can be filtered best. Radboud’s PhD students Sara Issaoun and Michael Janssen, and JIVE astronomer Ilse van Bemmel figured out how to automate the process, using scripts for the open community package CASA. The results are perfectly calibrated data. They are published in a separate article, of which Sara Issaoun is one of the head coordinators.

Such a calibration pipelines (automatic data processing) had been a dream of the VLBI community for 20 years. Falcke: “Three of the five researchers in the calibration group were Dutch. They have achieved amazing work. With such a pipeline, you can be sure that the data processing can be replicated perfectly. Something of that importance cannot depend on someone’s personal preference, you need to be able to test options and replicate them!”

It is the central theme of the Radboud contribution to the Event Horizon Telescope: inventing extreme clever calculation tools that can be used widely. An example is PhD student Freek Roelofs, who is building a simulator, together with colleagues in South Africa, based on the Dutch radio telescope LOFAR. The new SYMBA tool can predict how well the VLBI network can observe a source in the sky, down to the influence of the weather at the different telescopes: extremely useful to determine the margin of error in the measurements.

First image

Once the data were ready to be used, experts within the entire EHT project started making their own images. Radboud astrophysicist Sara Issaoun leaded one of the four imaging teams of EHT, which each had their own approach. She wrote an important part of the imaging analysis in the academic publication about the image of the black hole in M87.

Falcke remembers that “the first image of M87 I saw was made by our PhD student Freek Roelofs. I saw it just before a TEDx event, so you can imagine that I was walking around completely delighted, without being able to say anything!”

Close collaboration between observation and theory

In the photograph of the black hole, all kinds of expertise have been combined. For example, new data processing and measuring instruments for the eight telescopes in the EHT network, but also elaborate calculation models that predict what the measurements will look like. The theorist Monika Mościbrodzka, together with PhD students Thomas Bronzwaer and Jordy Davelaar and the BlackHoleCam team in Frankfurt, are an example of this. They worked closely together with the observers. “This allowed us to deduce all kinds of characteristics of the black hole on solid ground. Our efforts during the early stages of the EHT eventually led to such a strong theory group within the EHT.”

The team of Falcke and Mościbrodzka pulled out all stops for the simulations: Thomas Bronzwaer modelled how the black hole diverges the light of the surrounding glowing gas cloud, and his colleague Jordy Davelaar modelled how the charged gas moves and radiates in the intense gravitational field. “They exactly knew what type of signal we should expect”, says Falcke, and the other way around: “we could deduce the state of the black hole and its surroundings even better from the measurements.”

The BlackHoleCam also accounted for different models about gravity, and not just Einstein’s one: in 2020, the EHT can strongly confirm Einstein’s General Theory of Relativity – some different theories of gravity seem to not fit the measurements as well.

Plans for the future

After the first picture of the black hole in M87, Event Horizon Telescope is definitely not done yet. Using more measurements, astrophysicists want to make videos that show the change of black holes over time.

This is why Falcke started the centre for expertise Radboud Radio Lab in 2015, which is now collaborating with the University of Namibia on the new Africa Millimeter Telescope AMT. It is located in the perfect spot to close the gap in the EHT network, making for an even better signal coverage: it is the entrance ticket of The Netherlands and Europe in the future of black hole photography.

The funding has not been arranged yet: “a difficult process in the current times”, according to Falcke, “even if you are really successful, it does not mean that you receive a lot of money so you can compete with large countries like the USA.”

And next? Going to space, in order to make high resolution videos of black holes and the orbiting matter, using two satellite telescopes. ESA should be able to launch such a mission in 2050. Falcke hopes he lives to see the day: “This is way I am planting the seeds now, just like I did five years ago with the Africa Millimeter Telescope. Once the tree is growing and the first fruits can be reaped, we can enjoy it all together.”