SARAO News

Lerato Sebokolodi doesn’t like being told what to do. This independent thinker nearly left academia for the private sector, until she realised that she would have to dress a certain way, follow instructions and rules, and generally do what other people told her. Her choice to stay was a good thing for the discipline of astrophysics, because now she is doing groundbreaking work on one of the universe’s most-studied objects, a radio galaxy called Cygnus A.

Sebokolodi started her academic career studying physics and mathematics, ultimately hoping to go into geophysics. Then a lecture from visiting astronomer Thebe Medupe piqued her interest in astrophysics while she was completing her BSc.

“He told me what he was working on and I thought it sounded great,” she says. “He gave me an opportunity to attend a winter school, which gave me a much wider exposure to astronomy, and I decided to continue in the field.”

She completed her Honours degree through the National Astrophysics and Space Science Programme (NASSP) at the University of Cape Town (UCT), and then began to think about getting a job.

“I couldn’t see where I would fit in when I started applying for jobs, given my personality. I wanted the freedom to do what I want to do, not to follow instructions. It was so hard to find a job that suited me.”

Fortunately, her Honours project supervisor, introduced her to Prof Oleg Smirnov, SKA Chair in Radio Astronomy Techniques and Technologies at Rhodes University. She joined Smirnov’s team to work on data processing – her Masters degree involved developing an algorithm to determine what is real and not real in radio interferometry images.

“I knew I didn’t have the computer science background, but I wanted to do something new,” says Sebokolodi. “It went well, but by the end I wasn’t sure whether I wanted to continue with software-related work – I had diverged from my initial passion to study physics.”

Her next career step would put her firmly back on track.

A brief history of Cygnus A

In November 2016, Smirnov introduced Sebokolodi to Dr Rick Perley of the National Radio Astronomy Observatory (NRAO), who offered her a PhD working on Cygnus A. She accepted soon after and moved to the USA in August 2017.

Perley has been studying Cygnus A since 1977, when he took up the first ever post-doctoral position at the newly-established Very Large Array (VLA), located on the bed of an ancient lake near Socorro in the New Mexico desert. The VLA is the largest and most powerful radio telescope in the world, and will be until SKA Phase One comes online in 2023.

“The VLA was designed to focus on astrophysics problems of the time – one of these was the puzzle of radio galaxies and quasars,” says Perley. Radio galaxies like Cygnus A show two huge balloons, or lobes, of electromagnetic radiation in the low-energy frequency end of the spectrum – radio waves. These lobes are expelled by certain galaxies, but when the VLA was commissioned, that was about all that could be said about these celestial objects.

“We wanted to know what is fuelling the radio lobes, how did they evolve, what is the content? VLA gave us the sensitivity and resolution to look into the galaxy core, the hotspots, the jets. I chose to look at Cygnus A.”

Cygnus A is the best radio galaxy to study in the Northern hemisphere – it is relatively close and extremely bright, which means we can study it with high spatial resolution and good sensitivity. Not only this, but Cygnus A is highly linearly polarised – a source is polarised if its electromagnetic radiation has a prefered orientation and linear if the waves oscillate in a single direction.

Researchers learned much about the structure and physical characteristics of radio galaxies in general, and Cygnus A in particular, over that time. They now knew that at the centre of Cygnus A is a black hole, which accretes matter as all black holes do. However, some matter is expelled by the black hole as jets of radio energy, and interacts with interstellar medium which in turn creates a shockwave that generates the radio lobes that have been observed for this and other radio galaxies. They also found that at the extremities of the lobes of Cygnus A are two hotspots.

More importantly, they noticed that polarised emissions from Cygnus A, like many other radio galaxies, are rotated during propagation towards Earth.  By the early 1990s, after years of observations and research, Perley felt he and his colleagues had found a clear picture of the physics of Cygnus A and its environment, and moved on to other projects.

At this point, it was clear that the matter surrounding Cygnus A causes rotation of polarised emissions. However, researchers could not yet disprove that the observed rotation could be due to thermal emissions inside the radio lobes. Although they were able to measure the polarisation properties of Cygnus A, the measurements could only be made at a few frequencies, and investigating the contents of the lobes was close to impossible.

It would take a technological upgrade of the VLA before researchers could start to shine a light on these mysteries.

Carrying the radio astronomy torch

That upgrade came in the form of a $90 million technology injection into the VLA. Completed in 2012, Perley was closely involved in the process.

“Sometime during the 1990s, I became interested in promoting improvements to the telescope, and headed up the VLA upgrade project,” says Perley. “This has increased its capabilities in virtually every way except for resolution – new receivers, correlators, fibre optics and other technology.”

All of these improvements greatly expanded the possibilities of research on the telescope, known as the Karl G. Jansky Very Large Array (JVLA) since it was recommissioned in 2012. In particular, it meant that Cygnus A could be studied in ways that weren’t previously possible, providing the technical capabilities needed for Sobokolodi’s to do her work.

“The idea of working with people who have worked on Cygnus A for years, that was really appealing,” says Sebokolodi. “I couldn’t refuse the offer.”

Perley invited Sebokolodi to work on data from 44 hours of observations of Cygnus A, across a vastly expanded frequency spectrum – in the 1970s, Perley had collected radio frequency data across just four channels, but now researchers had literally thousands of channels to work with.

He cleaned and calibrated the data, and in August 2017 she moved to the tiny university town of Socorro, New Mexico, to start work in earnest.

Sebokolodi fitted in almost immediately, making friends and filling her days with social activities – no easy task in a town of only 8000 people, way out in the American Midwest.

Perley and Sebokolodi clearly found a good personality match in each other – she wants to be left to her own devices and really get stuck into a problem, and he likes to give his students free rein to solve their own problems, only stepping in occasionally to provide guidance or help them get past obstacles. In just five months since Sebokolodi has been working on Cygnus A, they already have exciting and potentially groundbreaking results.

The wealth of new frequency data has allowed Sebokolodi to revisit previous assumptions about certain characteristics of the radio galaxy. She has detected depolarisation in the radio signals, which indicates that the radio lobes can’t be empty – there must be some gas inside them.

“Lerato recently came back to me with surprising results,” says Perley. “They are not in line with my preconceived ideas about the polarisation structure of Cygnus A, which is a little embarrassing, but also extremely pleasing to me.”

“Overall, we’re very pleased with the progress. I think we’re going to make a pretty big splash when we make the results public.”

The future of radio astronomy is Southern

While these results are set to make waves right now, Sebokolodi is already looking to the future.

“Getting observation time on the SKA will be very competitive, so I have to know what I am talking about,” she explains. “I want to acquire as many skills as possible now. When you write proposals, the reviewers want to see that people involved in the proposal have the correct expertise.”

The SKA is mostly employing engineers and software developers at the moment, because the telescope is still under construction. But a few, like Sebokolodi, are being employed and funded to gain skills in basic science as part of the Young Professionals Development Programme at the SKA.

Sebokolodi says that at first she felt like she wasn’t making a contribution. “But look at the bigger picture – in the future I will be an expert in cluster magnetic fields, radio galaxies, and radio astronomy techniques. There is no better place for me to learn.”

A perfect example of this is the technique she is using to study Cygnus A, known as rotation measurement synthesis; it is a relatively new technique in this field, and she is putting it to novel use.

“Our methods and results will ultimately guide and inform work done at the SKA on similar objects,” explains Perley. “We’re establishing a path, but other people will need to build the roads to get the best out of the technique and the instruments they have.”

Sebokolodi is gracious in her excellence and grateful for the opportunity that has been presented to her.

“My days are spent researching and doing what I want to do. This research project allows me to have my own ideas, to question people’s assumptions, and to contribute to the field. I really am blessed right now.”