A study accepted in The Astrophysical Journal Letters reports the discovery of PSR J0125−5854, a fast-spinning pulsar found with the Murchison Widefield Array during the ongoing SMART survey of the southern sky. The object spins once every 24.6 milliseconds and appears to be part of a wide binary system with an orbit that may last about 833.6 days.
The finding matters because millisecond pulsars are among the most precise natural clocks in the universe. These dense stellar remnants can help astronomers test gravity, probe the material inside neutron stars and search for subtle ripples in spacetime. This one is especially valuable because it was found at low radio frequencies by a telescope built to scan huge areas of sky.
Led by Chia Min Tan of Curtin University, the team discovered the pulsar through the Southern-sky MWA Rapid Two-metre survey, known as SMART. The survey uses the Murchison Widefield Array in Western Australia to search the southern sky for pulsars and fast radio transients.
In the paper’s abstract, the researchers write, “We report the discovery of PSR J0125−5854, a pulsar with a spin period of 24 ms.” That compact statement points to a remarkable object, a stellar core with more mass than the Sun packed into a city-sized sphere and spinning dozens of times each second.
A fast-spinning neutron star in the southern sky
PSR J0125−5854 is a millisecond pulsar, which means it rotates in less than 30 milliseconds. Pulsars form when massive stars collapse into neutron stars. If their magnetic beams sweep across Earth, radio telescopes detect them as repeating flashes.
A 24.6-millisecond spin places this object in an energetic class of recycled neutron stars. Astronomers think many millisecond pulsars gained their speed in binary systems. Gas from a companion star can fall onto the neutron star and spin it up over long periods of time.
The new pulsar has a dispersion measure of 11.66 pc cm⁻³. Dispersion measure tracks how much free electron material the radio signal crosses on its way to Earth. That measurement helps astronomers estimate distance and study the thin plasma spread through the Milky Way.
Using Galactic electron density models, the team estimates that PSR J0125−5854 lies about 0.5 to 1 kiloparsec away. That corresponds to roughly 1,600 to 3,200 light-years. In cosmic terms, it is a relatively nearby addition to the catalog of known pulsars.
The pulsar also sits at a high Galactic latitude of about minus 57 degrees. That location places it well away from the crowded plane of the Milky Way, where many pulsar searches focus. Its discovery shows why wide-area surveys can still reveal compact objects in less obvious parts of the sky.
The first millisecond pulsar found with the Murchison Widefield Array
The discovery marks the first MWA millisecond pulsar identified by the Murchison Widefield Array. It is also the first pulsar found in the deep-pass searches of the SMART survey, according to the research team.
MWA is a low-frequency radio telescope located at the Murchison Radio-astronomy Observatory in Western Australia. Its strength comes from a huge field of view. Instead of staring at one narrow patch of sky, it can monitor broad regions and search for brief or repeating radio signals.
The SMART survey uses MWA’s voltage capture system to scan the sky south of 30 degrees declination. It works in the 140 to 170 MHz frequency band, a low-frequency range that can be especially useful for finding steep-spectrum pulsars. These objects appear brighter at lower radio frequencies.
PSR J0125−5854 has a steep radio spectrum, with the team reporting a spectral index near minus 2.2. In simple terms, its radio signal grows stronger toward lower frequencies. That makes it well suited to discovery by an instrument like MWA.
For astronomers, this is also a proof of method. The SMART survey has processed only part of its planned data. A millisecond pulsar appearing this early in the search suggests that more fast-spinning neutron stars could be waiting in the southern sky archive.
A binary system with an 833-day orbit
Follow-up observations with MWA and the MeerKAT radio telescope revealed that PSR J0125−5854 is likely in a binary system. The data show orbital motion across a long timescale. The team reports that the orbit is longer than 290 days and may be about 833.60 days.
That possible 833.6-day period makes the system unusually wide for a recycled pulsar binary. In a compact binary, two objects circle each other in hours or days. Here, the neutron star appears to take well over two years to complete one orbit around the system’s center of mass.
The current timing solution points to a projected semi-major axis of about 241.36 light-seconds. A light-second is the distance light travels in one second, about 186,000 miles. This value describes the apparent size of the pulsar’s orbit as inferred from changes in pulse arrival times.
The orbit also appears to have low eccentricity, with a reported value near 0.0052. That means the path is close to circular. Such a smooth orbit can carry clues about the system’s past, including earlier mass transfer between the companion star and the neutron star.
Pulsar timing makes these measurements possible. Astronomers record the arrival time of each pulse and track tiny shifts. When a pulsar moves toward Earth in its orbit, pulses arrive slightly earlier. When it moves away, pulses arrive slightly later. Over many observations, those shifts reveal the shape and scale of the binary system.
Why a helium white dwarf may be hiding there
The companion to PSR J0125−5854 has a minimum estimated mass of about 0.415 solar masses. That mass points toward a compact stellar remnant. The authors suggest that the companion is likely a helium white dwarf.
A helium white dwarf forms when a star loses its outer layers before it can build a heavier core. In many pulsar systems, this stripping happens through mass transfer. The future neutron star or the neutron star itself pulls material from the companion, reshaping both objects over time.
This history fits the broader picture of millisecond pulsar formation. A neutron star begins as a slower rotator. Over time, matter from a companion can transfer angular momentum to it. The neutron star spins faster and the donor star can become a white dwarf remnant.
The long orbit adds an intriguing twist. A wide pulsar-white dwarf pair may preserve a record of mass transfer that ended gently compared with more chaotic binary histories. The low eccentricity also supports a relatively settled system, though the researchers remain careful about the interpretation.
The team emphasizes that more timing data are needed. In the paper’s abstract, they state, “Further observations are required in order to better constrain the orbital and spin parameters.” Longer monitoring will help refine the orbit and test the helium white dwarf interpretation.
What the SMART survey could find next
The SMART survey is designed to search for pulsars and fast transients across the southern sky. Its combination of large sky coverage and low-frequency sensitivity gives it a distinctive role. It can locate objects that higher-frequency or narrower searches may miss.
Once complete, SMART is expected to discover hundreds of new pulsars. PSR J0125−5854 strengthens that expectation because it emerged from a limited portion of the search. The result hints at a larger population of low-frequency pulsars still buried in the data.
Future discoveries could include more binary pulsars, steep-spectrum pulsars, intermittent pulsars and objects with unusual spin behavior. Each one adds a new clock to the Galaxy. Together, they help map the Milky Way’s electron content and reveal how massive stars evolve after collapse.
The discovery also has implications for upcoming low-frequency surveys with SKA-Low, the low-frequency component of the Square Kilometre Array. MWA is a precursor instrument for the SKA project. Lessons from SMART processing can guide future searches across larger and more sensitive datasets.
For now, PSR J0125−5854 stands as a strong early result for the SMART survey. It is a fast-spinning neutron star, a probable wide binary and the first millisecond pulsar found with MWA. Its steady pulses may keep telling astronomers more as the timing baseline grows.
