Our Cosmic Neighbourhood Not So Empty: Powerful Telescope Detects Swirling Plasma

The discovery of 25 scintillation arcs – a record – reshapes our understanding of the interstellar medium, suggesting it's far more complex and dynamic than once believed

With the most powerful radio telescope in the southern hemisphere, we have observed a twinkling star and discovered an abundance of mysterious plasma structures in our cosmic neighbourhood. The plasma structures we see are variations in density or turbulence, akin to interstellar cyclones stirred up by energetic events in the galaxy.

The study, published today in Nature Astronomy, also describes the first measurements of plasma layers within an interstellar shock wave that surrounds a pulsar. We now realise our local interstellar medium is filled with these structures and our findings also include a rare phenomenon that will challenge theories of pulsar shock waves.

What's a pulsar and why does it have a shock wave?

Our observations honed in on the nearby fast-spinning pulsar, J0437-4715, which is 512 light-years away from Earth. A pulsar is a neutron star, a super-dense stellar remnant that produces beams of radio waves and an energetic “wind” of particles.

The pulsar and its wind move with supersonic speed through the interstellar medium – the stuff (gas, dust and plasma) between the stars. This creates a bow shock: a shock wave of heated gas that glows red.

The interstellar plasma is turbulent and scatters pulsar radio waves slightly away from a direct, straight line path. The scattered waves create a pattern of bright and dim patches that drifts over our radio telescopes as Earth, the pulsar and plasma all move through space.

From our vantage point, this causes the pulsar to twinkle, or “scintillate”. The effect is similar to how turbulence in Earth's atmosphere makes stars twinkle in the night sky. Pulsar scintillation gives us unique information about plasma structures that are too small and faint to be detected in any other way.

Twinkling little radio star

To the naked eye, the twinkling of a star might appear random. But for pulsars at least, there are hidden patterns. With the right techniques, we can uncover ordered shapes from the interference pattern, called scintillation arcs. They detail the locations and velocities of compact structures in the interstellar plasma.

Studying scintillation arcs is like performing a CT scan of the interstellar medium – each arc reveals a thin layer of plasma. Usually, scintillation arc studies uncover just one, or at most a handful of these arcs, giving a view of only the most extreme (densest or most turbulent) plasma structures in our galaxy.

Our scintillation arc study broke new ground by unveiling an unprecedented 25 scintillation arcs, the most plasma structures observed for any pulsar to date. The sensitivity of our study was only possible because of the close proximity of the pulsar (it's our nearest millisecond pulsar neighbour) and the large collecting area of the MeerKAT radio telescope in South Africa.

(The author is associated with Swinburne University of Technology)