A group of around 200 researchers recently succeeded in capturing the first ever image of a super massive black hole 55 million light years away.
They observed the object - which has a mass 6.5bn times more than our Sun - for five nights in April 2017 using eight ground-based telescopes located in different parts of the world. By linking these individual telescopes, located as far as Spain and Antarctica, they effectively created an Earth-sized aperture.
Astrophysicist Professor Roger Deane from the University of Pretoria led a small team charged with helping create a simulation of the array. Before scientists were able to conduct the actual experiment with this instrument, they needed to “understand its limits, and the extent to which we can make inferences about the shadow of a black hole”. This is where the South Africans came in.
It is important to understand that while the image is considered to be the first of a black hole, in actual fact, it is mostly of the object’s shadow. “The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape,” read a statement by the Event Horizon Telescope Consortium (EHT).
In order for the scientists to do this, a number of things needed to happen. Over a period of 10 years, “EHT data capture rates increased” 16 fold, significantly improving millimeter receivers - the ones used in the experiment observing at a wavelength of 1.3mm.
Scientists also needed to connect distant telescopes to create one ‘virtual telescope’. “This significantly boosts angular resolution,” the researchers wrote in one of six papers they published on the experiment. The combined capabilities of the virtual telescope is “enough to read a newspaper in New York from a sidewalk café in Paris”, EHT said.
Importantly, the researchers also had to ensure clear skies on the nights of observation.
Therefore Deane, along with postdoctoral researcher Iniyan Natarajan, and two graduate students from UP conducted the simulations using the eight existing sites to ensure the ultimate readings are accurate.
Deane said they were tasked with determining the conditions under which the actual observations could be made using software — MeqSilhouette — that measured a number of factors, including ground pressure, temperature and water vapour.
“For example, if the precipitable water vapour above a few of the EHT stations is above a certain value, will we still be able to image the shadow”, explained Deane, “or if the pointing accuracy of the certain antennas is not quite a specification, how does this affect image quality”.
To determine these factors, they faced, among others, the challenge of geographical scale. This, he said, means they needed to work with teams on different continents, across different time zones.
Then in April 2017, when all the simulations were done, the individual telescopes were directed at the centre of the galaxy Messier 87 and recorded roughly 350 terabytes of data each, every night during those five days in April. That data was then synchronised using atomic clocks (compensating for the earth's rotation and instrumental delays), stored on high performance hard drives, processed by specialised supercomputers and shipped to MIT in the US. “They were then painstakingly converted into an image using novel computational tools developed by the collaboration,” EHT said in its statement.
“It’s a very delicate experiment with a complex instrument,” Deane explained by email, “there are a multitude of imperfections that could wipe out the black hole shadow feature.” He said should only one part of the process be faulty, their reading could be distorted, “potentially leading to biased measurements”.
The combined effort resulted in “data of unprecedented scientific quality” that further present “exceptional opportunity for scientific discoveries”. EHT claims its capabilities are also set to improve going forward, including improved coverage and sharper resolution.
Three facilities have joined EHT since the experiment was conducted two years ago and more are expected to join the consortium. Deane said they are making the case for an African antenna to be included, specifically given the continent's geographical advantage and favourable weather conditions. He added an African telescope “would serve as a critical node in the network”.
By using the simulation the team from UP helped create, they will be able to “predict how well such a hypothetical array may perform and potentially guide its design”.
The software, of which the details will soon be published, will help determine where to place new stations, including, possibly one in Namibia.