The hunt for interstellar tornadoes with twinkling pulsars

Artist's illustration of a supernova

Artist's view of a supernova explosion, which drives turbulence in the interstellar medium. Credit: Carl Knox, OzGrav-Swinburne University

Space is mostly empty because of the vast distances between the stars that make up our Galaxy. Although it is often thought of as a vacuum, there is a diffuse medium that fills the void, made up of of gas, dust, and plasma (ionised particles). The plasma of this interstellar medium interacts with the radio waves that astronomers observe to study some of the most extreme objects in the universe, such as black holes and neutron stars (pulsars). In particular, the interstellar plasma slows down and scatters radio waves.

The observed result of this scattering is that radio waves from pulsars “twinkle” because of turbulence in the interstellar plasma, just as stars in the night sky twinkle because of turbulence in our atmosphere. Remarkably, astronomers can leverage this twinkling, also known as “scintillation,” to study the interstellar plasma and the pulsars themselves.

One mysterious feature of the interstellar plasma is the presence of dense, compact, and intensely-turbulent regions, akin to an interstellar tornado. These so-called “extreme scattering events,'' or ESEs, are poorly understood because they are difficult to study. Our current understanding is so poor that scientists would expect such extreme objects to quickly destroy themselves. How they form and how they sustain themselves is the mystery. The solution to this puzzle likely involves the magnetic fields in our Galaxy but further study of ESEs is critical. Unfortunately, ESEs are so small by astronomical standards, that they are completely invisible to other areas of astronomy.

A new research paper led by Dr Daniel Reardon of Swinburne University of Technology, offers a novel technique for finding changes in interstellar plasma density, caused for example by the passing of these interstellar storms through the radio waves from a pulsar. The technique involves identifying changes to the characteristics of the twinkling of pulsars as the plasma density varies. It can be used to measure the density and strength of such storms, which will provide valuable insights into their nature. Understanding ESEs and the magnetic fields that should support them, plays a role in our understanding of how galaxies themselves form, evolve, and birth stars.

The technique was proved through a practical demonstration on one pulsar, J1603-7202, which was known to have been affected by an ESE. The density of this ESE was previously estimated through the method of pulsar timing, which relies on detecting the time delay induced by the plasma, rather than the twinkling. The new method is complementary and broadens our window on these interstellar tornadoes. In this new study, a smaller structure in the interstellar plasma, similar to the known ESE, was also identified even though it was invisible to the pulsar timing method.

As we can now identify changes to the density of interstellar plasma using the twinkling of radio sources, many more pulsars can be used as tools to detect compact structures, such as ESEs. Pulsars are incredibly useful objects for studying the weather of interstellar space, and the weather in turn can be used to study the pulsars themselves. Pulsar timing and twinkling will be used together to solve fundamental problems in physics, such as the nature of gravity and the behaviour of matter at extreme density.

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