Unveiling Extreme Physics with the Galaxy’s Only Double Pulsar System

Throughout the Milky Way Galaxy, many stars exist in binary systems, a concept that might feel unfamiliar to us because our own Sun stands alone. Yet for astronomers, these stellar pairings are a goldmine. The gravitational influence each star exerts on the other allows scientists to uncover critical details about their orbits, masses, and more. 

Among these systems, one stands out as an extraordinarily rare and valuable astrophysical laboratory: the double pulsar system PSR J0737−3039A/B. This system features two neutron stars spinning rapidly and emitting beams of radio waves from their magnetic poles as they orbit a shared centre of mass.

What makes PSR J0737−3039A/B uniquely important is that it’s the only known double neutron star system where both pulsars’ radio beams are directed toward Earth. More so, and by pure cosmic coincidence - this double pulsar system is viewed almost perfectly edge-on from our perspective, which means that one pulsar eclipses the other. This perfect alignment offers astronomers an unprecedented opportunity to study the physics of extreme environments.

Now, a new study led by Dr. Marcus Lower from Swinburne University of Technology has taken this opportunity to a whole new level. By analysing how one pulsar’s radio waves pass through the magnetosphere of its companion, the team has gained fresh insights into the propagation of radio waves in this intense environment and mapped the geometry of the system from our vantage point.

Dr Lower is no stranger to the Double Pulsar system - studying it in his intern days and into his PhD under Swinburne University’s Prof. Matthew Bailes. “He [Prof. Bailes] showed me an absolutely jaw-dropping animation comparing a simulation of the eclipses against data collected by the Green Bank Telescope.” 

“There were plans to study these eclipses with MeerKAT, a new radio telescope that had just finished construction in South Africa. And when the opportunity arose, I leapt at the chance to be the leader of this project,” he said.

The MeerKAT telescope is a radio interferometer located in the Karoo region of South Africa. It consists of 64 dishes working together, achieving high sensitivity and optimal resolution, making it ideal for studying radio wavelength phenomena such as galaxy formation, pulsars, and the large-scale structure of the universe. 

“MeerKAT is the best telescope in the world for studying the double pulsar. The large collecting area of its 64 antennas and its location in the radio-quiet Karoo Desert gave us an unprecedented view of the eclipses in this system.”

PSR J0737−3039A/B was first discovered in 2003 with Murriyang (CSIRO’s Parkes radio telescope) by Dr. Marta Burgay as part of the Parkes High-Latitude Pulsar Survey. This discovery stands as one of the most remarkable in astrophysics, offering a rare opportunity to use pulses from both neutron stars to probe gravitational and electromagnetic phenomena.

The fact that this double pulsar system remained bound is a marvel in itself, given the violent nature of its formation. It began as a pair of massive stars in a binary orbit, with the first evolving faster and eventually exploding in a supernova. Typically, such an explosion would disrupt the system, sending one star hurtling away. But in a twist of fate, the binary remained intact, with the newly formed neutron star and its massive companion still gravitationally bound.

As the surviving massive star aged, it expanded, shedding its outer layers. The neutron star’s strong gravitational pull began siphoning this material, transferring angular momentum and spinning the neutron star up to incredible speeds, transforming it into a millisecond pulsar. The two stars were drawn even closer in their orbit until, eventually, the second star also went supernova. Remarkably, the system survived yet again, leaving behind two pulsars locked in an orbital dance.

“The entire process of forming the double pulsar, from the birth of their progenitor stars to the supernova that formed pulsar B would've only taken a few million years,” said Dr Lower.

The pulsars orbit their common centre of mass every 2.4 hours, and from Earth, we are fortunate to see this near edge-on alignment. This vantage point allows astronomers, like Dr. Lower, to observe the pulsars eclipsing one another, providing a unique opportunity to study their interactions. Pulsar A, the millisecond pulsar, spins rapidly, completing a rotation every 22.7 milliseconds after being spun up by accreted material from its companion. Pulsar B, by contrast, rotates more slowly at 2.77 seconds per rotation—still remarkably fast but noticeably different from its partner.

The double pulsar system’s orbit is almost perfectly edge-on from our vantage point on Earth, giving us a near-horizontal view. This rare alignment is crucial for studying the eclipses of pulsar A by pulsar B, offering a unique opportunity to observe how pulsar B’s magnetic field influences the light emitted from pulsar A.

“The eclipses allow us to shine a light through the powerful magnetic field of a pulsar, directly illuminating the extreme plasma physics that takes place around these objects.”

Such precise alignment allows for high-resolution measurements of the system’s geometry, magnetic field structure, and polarisation effects. The eclipses themselves, especially how the polarisation of radio waves from pulsar A changes as they pass through pulsar B's magnetosphere, were central to Dr. Lower’s research.

“The eclipses only last 40 seconds so we had to be quite careful in timing our observations.”

“Even back when pulsar B could be detected* it was only ever seen at very particular parts of its orbit,” he said. “This is because pulsar A is quite a bully. It emits a powerful wind of high-energy particles that blows away much of pulsar B's magnetic field.”

“Oh, and we haven't seen pulsar B since 2008 due to relativistic precession shifting its radio beam beyond our line of sight.”

Considering the rarity of this alignment, it raises questions about potential observational biases in our detection of other binary pulsar systems. Could there be more of these systems out there, hidden from us because their beams don’t shine in our direction or because they’re obscured by the dense stellar environments in places like the Galactic Center?

“The fact we've already found one double pulsar means there should be plenty more out there. We just have to be incredibly lucky for both pulsar beams to be pointed at us. There's a good chance that next-generation radio telescopes like the Square Kilometer Array will find them, though.”

A key finding of this study centres on how the radio waves from Pulsar A interact with the magnetic field of Pulsar B as they travel to our observatories. Pulsar signals are known for their high polarisation, which refers to the orientation of the electromagnetic wave’s plane. Polarisation can be either linear — oscillating up and down in a single plane — or circular, spiralling like a corkscrew. Imagine playing with a garden hose: if you moved your hand up and down, the wave would travel linearly, but if you made a circular motion, it would spiral.

When polarised light passes through a magnetised medium, the plane of its orientation can shift — a phenomenon called Faraday Rotation. By measuring these changes in pulsar signals, scientists can learn about the magnetic fields that influence the light on its journey.

In this study, Pulsar B’s powerful magnetic field acts almost like a filter for the radio waves emitted by Pulsar A. When the radio waves of Pulsar A pass through Pulsar B's magnetosphere, they undergo significant polarisation effects, including circular polarisation and rapid shifts in the linear polarisation angle. These observations confirm that the eclipses are caused by synchrotron absorption within Pulsar B’s relativistic pair-plasma (the surrounding field of plasma associated with its magnetosphere). This insight allows scientists to map the magnetic field and viewing geometry of Pulsar B with exceptional precision.

“We were initially quite surprised by how much circular polarisation was detected,” said Dr Lower. . “Though it turned out to be an almost perfect match to our simulations of radio waves interacting with a soup of super-heated matter and anti-matter particles.”

“We also found that the switching of the circular polarisation between spiralling clockwise to anti-clockwise and then back again was linked to the direction of pulsar B's magnetic field at different phases of the eclipses.”

“This is the first time we've been able to unambiguously determine the full viewing and magnetic geometry of a pulsar. It opens the door to even more detailed studies of this incredible system and others through better-informed supercomputer simulations, something that is much needed ahead of future studies of double pulsars with the SKA.”

The PSR J0737−3039A/B system serves as a remarkable natural laboratory for exploring extreme gravity — all from the safe distance of Earth. Neutron stars are incredibly dense, each containing about 1.4 times the mass of our Sun compressed into a sphere roughly 22 kilometres across. In this double neutron star system, the gravitational fields are so intense that they create the perfect environment to test Einstein’s theory of General Relativity, our most accurate understanding of gravity. Scientists can observe relativistic effects like gravitational time dilation, orbital decay, and spin precession with extraordinary precision.

This isn't new territory for PSR J0737−3039A/B. In 2021, a comprehensive study using 16 years of data confirmed strong support for General Relativity. Researchers even discovered that the two neutron stars are spiralling toward each other, radiating away energy as gravitational waves and inching closer by about 7mm per year. At this rate, they will merge in approximately 85 million years — a cosmic dance destined for a spectacular finale.

The system’s precise pulsar signals, akin to having two atomic clocks in orbit, have also allowed scientists to place incredibly tight constraints on the masses of each neutron star. By monitoring for any subtle variations in these pulses, researchers continue to push the boundaries of our understanding of fundamental forces, probing the limits of physics in some of the universe's most extreme conditions.
 

Video Credit: John Rowe Animations / SolarParallax YouTube