Shock insights – why objects in the radio sky twinkle

It’s one of the first things any of us learn about astronomy – stars twinkle while planets don’t. However, other point-like objects in the radio sky also twinkle, or “scintillate,” including spinning neutron stars known as pulsars. A team led by Australian scientists has used a scintillating pulsar to perform a CT scan of the interstellar medium in our galaxy, mapping previously unseen layers of plasma, including within a rare structure called a bow shock.

The discoveries, published today in Nature Astronomy, challenge existing theories of our local interstellar medium and will lead to new models for pulsar bow shocks.

The study, led by Dr Daniel Reardon from the ARC Centre of Excellence for Gravitational Wave Discovery and Swinburne University of Technology, is the culmination of over six days of observing the nearest and brightest millisecond pulsar to Earth using the MeerKAT telescope in South Africa, the most powerful radio telescope in the Southern Hemisphere.

While pulsars emit radio waves rather than visible light (like stars), they still “twinkle” because of turbulence in the plasma that exists in the space between stars. “This plasma is created from gas that is heated and stirred up by energetic events in our galaxy, like exploding stars,” Dr Reardon said.

“When a pulsar scintillates, it reveals valuable information about the location, structure, and motion of the plasma, as well as about the dynamics of the pulsar—we use scintillation to get unique insights about these interstellar storms.”

The pulsar in question, unimaginatively named J0437-4715, is located relatively close to our solar system, in an area of our galaxy called the Local Bubble—a region almost devoid of gas and dust, created by the explosions of 15 stars about 14 million years ago.

Using the data gleaned from MeerKAT, the scientists studied patterns called “scintillation arcs,” which provide a three-dimensional map, of plasma structures in the galaxy that are impossible to study using other methods. “These scintillation arcs revealed an unexpected abundance of compact solar-system sized blobs of plasma within our Local Bubble, which was thought to be more smooth,” Dr. Reardon said.

For the first time, the team also used scintillation to study the bow shock created by the pulsar as it ploughs supersonically through the interstellar medium. “Travelling at Mach 10, the pulsar and its energetic wind of fast-moving particles create a shock wave of heated gas.” The shock is akin to the bow wave at the front of a ship.

While most pulsars should create bow shocks, only about a dozen have ever been observed as a faint red glow of energised Hydrogen atoms. This study marks the first time scientists have been able to peer inside a pulsar bow shock to measure plasma speeds. “To our surprise, the scintillation arcs revealed multiple sheets of plasma inside the shock, including one unexpectedly moving towards the front of the shock," Dr Reardon said.

This groundbreaking study, made possible by the pulsar’s closeness to Earth and the power of the MeerKAT telescope, achieved several significant firsts including a measurement of the three-dimensional shape of a bow shock, measurement of plasma speeds inside the shock, and the most detailed view of plasma structures within our Local Bubble. “We can learn a lot from a twinkling pulsar!”