Twinkle, Twinkle, Radio Star

Have you ever noticed that some stars appear to twinkle when we look up at them at night? It’s the reason why many of us are taught that nursery rhyme as children. But to be honest, it’s always been a lie. 

The stars themselves do not twinkle (that’s not to say they don’t have variability), but instead, it is the effect of starlight passing through our turbulent atmosphere, in what is known as scintillation. 

Stars are tiny points of light that are so far away that our atmosphere causes their position, brightness and even colour to appear to scatter around. The same thing occurs with planets, but because they are much closer, the effect is less noticeable. 

Scintillation doesn’t just affect the visible light that we see with our eyes, or with our telescopes. It can also change the way we see radio waves from different sources in space. 

Now, a new research paper, led by Curtin University Ph.D. student Kathryn (Kat) Ross in Western Australia has produced some surprising results by observing the variability and scintillation of distant radio sources (like distant galaxies) whilst also considering what it could be that could be causing this.

“Originally we began this research as a check to make sure there wasn't much variability going on. The general assumption before this work was that the low frequency sky wouldn't show much variability on short (roughly year-long) timescales,” said Kat. 

The paper, published in the Monthly Notices of the Royal Astronomical Society, is the first of its kind to look at a large population of radio sources in the low-frequency range for spectral variability (that is, variability in their frequency) using data from the GaLactic and Extragalactic All-Sky MWA (GLEAM) Survey. 

“When I started looking into the survey that assumption didn't seem to hold up, and what scientist can refuse to investigate when they find something acting weird!” she said. 

Surprisingly (and excitingly), Kat and her team found that some of these radio sources - these distant, supermassive blackholes - varied on a scale of one Earth year, which piqued the team’s attention. How could something as big as this have its frequency (and light) jump around in a one year period (this timeframe is minuscule when considering its an the scale of the objects).

In astronomy, a radio source is an object in the Universe that emits radio waves. This is not like regular thermal radiation emissions, such as the processes which produce visible light or UV light from stars, but rather through non-thermal processes, when particles are accelerated to give off photons. 

A very simple example of this is the neutral hydrogen line. When large massive reserves of cold neutral hydrogen gas exist in space, they emit photons with 21cm wavelength (which is a radio wave at frequency 1.420 GHz). This is not due to any thermal processes, but rather a quirky flip of the electron spin direction in each hydrogen atom. 

Some of the strongest sources of astronomical radio waves include pulsars, some nebulae, quasars, and radio galaxies. Like the normal visible light waves we see, radio waves are still a part of the electromagnetic spectrum - only with a longer (bigger) wavelength. And whilst our eyes have evolved to absorb and detect the visible light portion of the spectrum (as well as the optical telescopes we have built), radio waves are not visible to us - and often require very big telescopes and arrays to detect them. 

Kat’s research in particular, focused on low-frequency wavelengths, between 100 and 230 MHz (megahertz) - hertz being the measure of cycles per second (and often used when discussing radio frequencies). At these frequencies, the radio waves measure a couple of metres from crest peak to crest peak.  

The data for this research comes from the GLEAM survey, which uses the Murchison Widefield Array (MWA) Telescope, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia, situated on the land of the traditional custodians, the Wajarri Yamatji people. The MRO is owned and operated by Australia’s national science agency, CSIRO. 

Unlike conventional telescopes, both optical and radio dish antennas, the MWA is a series of spider-like dipole antennas, arranged in a 4 x 4 grid model, sitting atop a 4m x 4m mesh. This compact formation is called a ‘station’ and the array is as it sounds - an array of these stations spread over a large distance, in outback Western Australia.

The GLEAM survey covers a large area of the sky - which includes everything south of +30-degrees declination, observing between the ranges of 72 and 231 MHz in five continuous bands (and excluding the known 137 MHz narrowband, which is reserved for satellite communications)  

As part of this research, Kat and her team used data from two different time-spans stretching across 2013 and 2014 to develop an understanding of the variability of radio sources within this low-frequency range. In other words, by looking at the differences in the two periods - any subtle changes that were occurring by the radio source could be noted. 

“It definitely would've been ideal to have more observations during the year rather than the two. However, since finding this, we have conducted exactly this kind of follow up campaign. We are observing a handful of variable sources multiple times over a year using both the MWA and the Australian Telescope Compact Array (simultaneously) so we can create a full Spectral Energy Distribution from 72MHz to 10GHz for each individual epoch. We're hoping this can provide some insight into what is going on.” 

To consider what might be causing this scintillation, it’s a good idea to better understand what kinds of variation radio sources exhibit: intrinsic and extrinsic. 

Intrinsic variation is caused by changes within the radio source itself - such as the supermassive black hole devouring a nearby star and emitting a lot of energy across the electromagnetic spectrum as a result. Whereas an extrinsic variation can be related to changes in the radio waves due to the medium they are travelling through, such as a lot of diffuse dust and gas in the Milky Way (or beyond) which the radio waves have to fight through before reaching Earth. Both of these types of variations can give us lots of information about the structure of the source, the environment around it, and some of the media that lie in our line of sight. 

Kat’s analysis of the data revealed that out of 21,000+ sources, 323 of these objects showed significant variation. These sources showed two different types of variation - a uniform variation across the epochs, and a change in spectral shape. 

Of the 323 exhibiting this variation, 51 showed a definitive change in shape, indicating potentially a blazar - a type of galaxy with jets of ionised matter streaming for the core in opposite directions, with one of the jets pointing towards the observer (us). 

On the other hand, uniform variation demonstrates interstellar scintillation, an interaction between the interstellar medium and the radio waves as they travel through it. 

“This paper shows the low-frequency sky is far more dynamic than we expected. With the next generation of telescopes, like the SKA coming online, surveys like this will become readily available. This survey marks the beginning of hopefully a bright future of spectral variability surveys,” Kat said.

“We are also following up 3 particularly interesting sources with the LBA (Long Baseline Array) to understand their small morphology This will hopefully allow us to detect small structures like knots in the lobes or check if they're blazars,” she continued.

There’s still a lot more to uncover about these exciting new findings, like pinpointing the possible causes of these variations at such small time scales (from a galactic perspective), and research like this, produced by Kat and her team will help shape future large population surveys of low-frequency radio sources - such as those that are planned by the big mega-science project, the Square Kilometre Array.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site.