Closing in on Earth’s Closest Millisecond Pulsar

Australian astronomers have used 26 years of data collected by Murriyang (the CSIRO Parkes radio telescope) to make precise measurements of the mass and distance of Earth’s closest and brightest millisecond pulsar, PSR J0437-4715. The new data, which have been used by X-ray astronomers using a NASA telescope aboard the International Space Station will now help place constraints on the densities of neutron stars, helping scientists understand how matter behaves under these extreme conditions. 

The study of this pulsar at radio wavelengths, which was led by Dr Daniel Reardon from Swinburne University of Technology and part of the Parkes Pulsar Timing Array project, has improved the precision of these measurements, placing the mass of the neutron star to be 1.418 +/- 0.044 solar masses, which is about the average value of neutron stars. The distance measured for PSR J0437-4715 from this new data is 511.93 +/- 0.35 light-years away from Earth. Also reported in the paper is the orbital inclination angle of 137.506 +/- 0.016 degrees.

Whilst this makes PSR J0437-4715 the closest millisecond pulsar to Earth, other neutron stars reside a little closer (such as RX J185635-3754 which is the closest thermally measured neutron star at 400 light-years distance, and a handful of classical, younger pulsars which reside approximately 300-400 light-years from Earth according to the CSIRO ATNF Pulsar Catalogue).

To demonstrate the robustness of these new data, the team of astronomers critically analysed different data sets collected from Murriyang over the last two decades. What they were measuring was the pulsar’s signal time of arrival (ToA), which includes information that describes any intrinsic effects the pulsar experiences (such as rotational irregularities, or profile changes) and information about the interstellar medium that the pulsar’s signal has to travel through before arriving at Earth. Along with this, these ToAs can highlight any instrumental issues or any noise parameters that need to be accounted for. 

By looking at a variety of data sets, and factoring in different noise models, astronomers were able to come to the new precise measurements of mass, distance and inclination angle.  

These updated data points on PSR J0437-4715’s mass, distance and orbital inclination angle are used to model the pulsar’s profile as seen by NASA’s X-ray NICER (Neutron Star Interior Composition ExploreR) instrument aboard the International Space Station. 

Whilst neutron stars have a lot of mass (usually about 1.4 times that of our Sun), all of this is compressed into an area no bigger than a small city or town, of roughly 20 km in diameter. What the NICER team is trying to measure is the neutron star equation of state which describes how matter behaves under the extreme gravity that neutron stars possess, as well as its structural form, temperature and fluidity when it is crushed at mind-boggling densities, within the interior of the neutron star. 

The data obtained from each telescope - one being a 64m diameter dish antenna that collects radio waves, and the other being a 1m cubed X-ray telescope located in space, allow scientists to study astrophysical phenomena, objects and events across the electromagnetic spectrum.  

Neutron stars and pulsars exhibit properties that cannot be replicated in terrestrial laboratories due to the extreme gravitational and density conditions that exist near or within these objects. This research exemplifies the collaborative potential between scientists studying various electromagnetic and gravitational spectrum regimes.

In two recent publications, researchers utilising radio measurements from Dr. Reardon's study and data from the NICER mission demonstrated how this comprehensive dataset enabled them to place tighter constraints on the mass, radius, and equation of state of PSR J0437-4715.

The first paper reports that the pulsar's mass is inferred to be 1.418 ± 0.037 times that of the Sun, with a radius of approximately 11.36 km. This determination allows for the computation of density by dividing mass by volume, suggesting a 'softer' equation of state for matter, which aligns with independent results observed from binary neutron star mergers. Additionally, this study provided improved constraints on a bright background X-ray source (an Active Galactic Nucleus) near PSR J0437-4715, as seen from Earth.

The second paper further examined the implications of this data on various models of the equation of state of matter and explored how these findings impact the mass and radius of neutron stars. This holistic approach underscores the importance of interdisciplinary collaboration in advancing our understanding of these enigmatic celestial objects.

Neutron stars are formed when a massive star (usually around 8 - 25 solar masses) can no longer sustain the nuclear burning of elements like hydrogen in its core. Usually, this produces radiation pressure through nuclear fusion processes creating enough radiation pressure to balance the inward pull of gravity, and so the star remains in equilibrium. However, as massive stars age and burn through their hydrogen reserves, this radiation pressure dwindles until a point at which gravity wins.

This causes the core of the star to collapse in on itself, triggering a violent supernova explosion. Whilst most of the outer layers of the star are thrown off into space when this happens, the core further compresses - forcing electrons and protons together to form neutrons, thus leaving behind a neutron star. 

During this collapse process, the conservation of angular momentum causes the newly formed neutron star to rotate rapidly, and the conservation of magnetic flux from the progenitor star state generates a powerful magnetic field. As a result of these two factors, the neutron star emits beams of radio wavelength radiation from its magnetic poles, and as the star rotates and these beams come into view, we detect them as pulses - hence why they are called pulsars. 

Classical pulsars (or young pulsars) rotate on their axis several times per second on average. Eventually, they dump a lot of their magnetic dipole energy into their surrounding environments and due to the conservation of energy, end up slowing down and having weaker magnetic fields. 

At a certain point, the pulses switch off because the star is no longer rapidly rotating, nor has a strong enough magnetic field to generate pulse emissions. The pulsar at this point, becomes a thermally cooling neutron star.

However, there is a sub-category of these older pulsars which have lower-strength magnetic fields, and yet are rotating even more rapidly - hundreds of times per second on their axis. These are the millisecond pulsar population, and the reason why they are rotating so fast is because they have been spun back up through the accretion of material from a binary companion. Effectively, the pulsar engine restarts as the pulsar steals material from its companion, transferring the angular momentum. Most millisecond pulsars form part of a binary system (or once did). 

PSR J0437-4715 was discovered in 1993 using CSIRO’s Parkes radio telescope and features not only the millisecond pulsar but a white dwarf binary companion whose mass is determined to be approximately 0.22 solar units. The pair orbits around a common centre of mass every 5.74 days. Due to its proximity to Earth, its brightness at radio frequencies and the long timing baselines that astronomers have accumulated, PSR J0437-4715 is also a primary target for many pulsar timing experiments, including playing a key role in the search for nanoHertz-regime (low-frequency) gravitational-wave backgrounds, with strong evidence for this recently reported.

PSR J0437-4715 pulsar emission regions animation credit: NASA/Sharon Morsink/Devarshi Choudhury et al.

PSR J0437-4715 pulsar and white dwarf video animation credit: Credit: Carl Knox / OzGrav.