By analyzing a twinkling pulsar, scientists have mapped plasma plumes and a bow shock wave with unprecedented clarity, upending the assumption that our local interstellar neighborhood is calm.
One of the first things people learn in astronomy is that stars twinkle, while planets don't. But twinkling isn't limited to visible light—there are objects in the radio sky that appear to twinkle or "twinkle." Among them are pulsars, rapidly spinning neutron stars that emit beams of radio waves.
A team of scientists in Australia has used the flickering signal from a nearby pulsar to study the structure of the interstellar medium—the space between stars—in our galaxy. By analyzing how the pulsar's radio waves flicker, they were able to map previously hidden layers of plasma, including formations within a rare, turbulent structure called a bow shock, where fast-moving stellar winds collide with the surrounding space.
The new study challenges long-held ideas about the structure of the local interstellar medium, the space in the immediate vicinity of our solar system, and offers new insights that may reshape models of pulsar bow shock waves.
The findings come from research led by Dr. Daniel Reardon. For six days, the team observed the brightest and closest millisecond pulsar known to Earth using the MeerKAT radio telescope in South Africa, the most sensitive of its kind in the Southern Hemisphere.
Although pulsars don't glow in visible light like regular stars, they emit radio waves that appear to "sparkle" as they pass through the turbulent plasma that fills the space between stars. "This plasma is created by gas heated and stirred by energetic events in our galaxy, such as exploding stars," Dr. Reardon explained.
"When a pulsar twinkles, it reveals valuable information about the location, structure, and motion of the plasma, as well as the dynamics of the pulsar – we use the twinkling to gain unique insights into these interstellar storms."
The pulsar in question, whose uninspiring name is J0437-4715, is located relatively close to our solar system, in a region of our galaxy called the Local Bubble – a region almost free of gas and dust, created by the explosions of 15 stars about 14 million years ago.
Using data collected from MeerKAT, the researchers studied patterns called "scintillation arcs," which provide a three-dimensional map of plasma structures in the galaxy that is impossible to study with other methods. "These scintillation arcs revealed an unexpected abundance of compact, solar-system-sized clumps of plasma within our local bubble, which was thought to be smoother," said Dr. Reardon.
For the first time, the team also used the scintillation to study the arc shock wave created by the pulsar as it travels at supersonic speeds through the interstellar medium. "The pulsar, moving at Mach 10 and the energetic wind of fast-moving particles, creates a shock wave of hot gas." The shock wave is similar to the bow wave at the front of a ship.
Most pulsars produce arc shock waves, but only about a dozen have ever been observed as a faint red glow of excited hydrogen atoms. For the first time, scientists were able to peer inside a pulsar arc shock wave to measure plasma velocities. To their surprise, the arcs revealed multiple sheets of plasma within the shock wave, including one that was moving unexpectedly toward the shock front.
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