🔭 astrophysics

Discovery of a 24-millisecond pulsar in a very long orbit with the Murchison Widefield Array

The Murchison Widefield Array's SMART survey has discovered PSR J0125$-$5854, a 24-ms millisecond pulsar in a highly eccentric, ultra-long binary orbit (potentially ~834 days) with a low-mass Helium white dwarf companion, highlighting the survey's potential to reveal rare pulsar systems and informing future low-frequency SKA-Low searches.

Original authors: Chia Min Tan, N. D. Ramesh Bhat, Bradley W. Meyers, Christopher P. Lee, Ewan D. Barr, Vivek Venkatraman Krishnan, Paulo C. C. Freire, Garvit Grover, Samuel J. McSweeney, Nicholas A. Swainston, Qiuyang

Published 2026-06-18

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Original authors: Chia Min Tan, N. D. Ramesh Bhat, Bradley W. Meyers, Christopher P. Lee, Ewan D. Barr, Vivek Venkatraman Krishnan, Paulo C. C. Freire, Garvit Grover, Samuel J. McSweeney, Nicholas A. Swainston, Qiuyang Fu, Mengyao Xue

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Discovery: Finding a Cosmic "Ghost" in the Dark

Imagine the night sky as a giant, crowded dance floor. For decades, astronomers have been looking for specific dancers: pulsars. These are the remnants of exploded stars (neutron stars) that spin incredibly fast, shooting out beams of radio light like lighthouse beams. Most of the famous ones are either slow, heavy dancers or super-fast "millisecond" dancers that have been spun up by a partner.

A team of astronomers using a massive radio telescope array in Western Australia called the Murchison Widefield Array (MWA) found a new dancer: PSR J0125−5854.

Here is the simple breakdown of what they found and why it's special:

1. The "24-Second" Beat

This pulsar spins 40 times every second (a 24-millisecond period). It's a "millisecond pulsar," which means it's an old, recycled star that has been spun up to high speeds. However, it's unique because it was found using low-frequency radio waves (like tuning into a deep bass station), whereas most pulsars are found on higher-frequency stations. It's like finding a rare instrument playing a deep, low note in a symphony where everyone else is playing high-pitched flutes.

2. The "Long-Distance" Relationship

The most surprising thing about this pulsar is its partner.

The Analogy: Imagine a couple dancing together. Usually, if they are dancing a fast waltz, they hold hands very close. If they are dancing a slow, lazy sway, they might be a bit further apart.
The Reality: This pulsar is in a binary system (a pair). Its partner is a Helium White Dwarf (a dead, cooling star). But they are very far apart. Their orbit takes 833 days (over two years) to complete one full circle.
Why it matters: Most millisecond pulsars have partners they hug tightly. Finding one with a partner it sees only once every two years is like finding a couple who only meet once a year for a dance, yet somehow, the dancer is still spinning super fast. This suggests the "spin-up" process happened a long time ago and the pair has since drifted far apart.

3. The "Chameleon" Problem

When the astronomers first found this pulsar, they thought they had a clear picture. But as they watched it over time, the pulsar's "heartbeat" (its spin period) seemed to speed up and slow down in a weird way.

The Analogy: Imagine you are watching a runner on a track. Sometimes they look like they are running fast, sometimes slow. You realize they aren't changing their speed; they are running on a giant, invisible merry-go-round. When they run toward you, they look faster (Doppler effect); when they run away, they look slower.
The Result: The team realized the pulsar was on a giant merry-go-round (its orbit). By measuring these tiny speed changes over months, they could calculate the size of the orbit and the mass of the invisible partner.

4. The "Invisible" Partner

The partner is a Helium White Dwarf.

The Analogy: Think of a white dwarf as a glowing ember from a campfire that has mostly burned out. It's hot but dim.
The Hunt: The astronomers looked for this partner using optical telescopes (like giant cameras). They found a faint, dim dot of light right where the pulsar's calculations said the partner should be. It's too dim to be a normal star, but just bright enough to be a white dwarf. This confirmed the "ghost" partner was real.

5. Why the "Low Frequency" Matters

This discovery is a big deal for the future of astronomy.

The Analogy: For years, astronomers have been looking for these fast-spinning stars using high-frequency radio waves (like FM radio). They missed this one because it's a "bass-heavy" star. It's very faint at high frequencies but shines brightly at low frequencies (like AM radio).
The Implication: The MWA telescope is designed to listen to these low frequencies. Because this pulsar has a very steep "spectrum" (it gets much dimmer as you go up in pitch), previous surveys using high-frequency telescopes (like the Parkes telescope) likely missed it. It was hiding in plain sight, waiting for the right "ear" to hear it.

The Bottom Line

The team discovered a fast-spinning dead star (PSR J0125−5854) that is dancing with a distant, dim partner (a Helium White Dwarf) in a very long, slow orbit.

This discovery proves that:

Low-frequency telescopes are essential for finding new types of pulsars that high-frequency telescopes miss.
There are likely many more of these "long-distance" binary pulsars out there, waiting to be found as the survey continues.
The universe is full of diverse relationships between stars, from tight hugs to long-distance dances, and we are just starting to map them all.

The paper concludes that this is just the beginning. As they process more data from the MWA and future telescopes (like the SKA-Low), they expect to find many more of these unique cosmic couples.

Technical Summary: Discovery of PSR J0125−5854

Problem and Context
Despite extensive pulsar surveys over the past decades, the population of known pulsars remains dominated by canonical (non-recycled) and recycled millisecond pulsars (MSPs). Intermediate-period pulsars (30–100 ms) and MSPs in very long-period binary systems remain underrepresented in current catalogs. Specifically, the detection of MSPs at low radio frequencies (<200 MHz) has been challenging due to sensitivity limitations and the prevalence of radio frequency interference (RFI). Furthermore, while the Murchison Widefield Array (MWA) has conducted the Southern-sky MWA Rapid Two-metre (SMART) survey, no MSP had been confirmed from its deep-pass (full 80-minute observation) searches prior to this work. The discovery of pulsars in wide binary orbits is crucial for understanding stellar evolution channels, particularly the formation of pulsar–white dwarf systems and the constraints they place on the equation of state of ultra-dense matter.

Methodology
The authors report the discovery of PSR J0125−5854 using data from the ongoing SMART survey. The discovery was made in an 80-minute observation taken on October 22, 2018, centered at a specific sky position. The raw voltage data were beamformed and searched using the Fourier-domain search process in the PRESTO software suite.

Following the initial detection, the team employed a multi-faceted follow-up strategy:

Localization: An improved position was derived by beamforming MWA voltage capture system (VCS) data on a hexagonal grid and analyzing signal-to-noise ratios (S/N) across adjacent beams. This was refined using MeerKAT observations in the UHF, L-band, and S1 bands, utilizing the SeeKAT package for maximum-likelihood estimation of the position.
Timing and Orbital Analysis: The team analyzed archival MWA data and new follow-up observations from the MWA (2024–2025) and MeerKAT (2025). They observed a significant variation in the spin period over time, inconsistent with simple spin-down, indicating a binary system.
- Initial orbital constraints were derived using a Markov Chain Monte Carlo (MCMC) fit to period measurements from the 2024–2025 epoch, assuming a Keplerian orbit.
- These parameters were further refined by modeling the times-of-arrival (TOAs) using the tempo software package, incorporating arbitrary jumps between epochs to account for phase incoherence.
Spectral Analysis: Flux densities were measured across a wide frequency range (~150 MHz to ~2.8 GHz) using MWA and MeerKAT data. The spectrum was modeled as a power law ( $S \propto \nu^\alpha$ ) using the pulsar spectra package, employing a Huber loss function to mitigate biases from scintillation.
Companion Identification: The authors compared the derived orbital parameters (period, eccentricity, minimum companion mass) against known binary pulsar populations and theoretical models for Helium white dwarf (He WD) formation (Tauris & Savonije 1999). They also searched for optical counterparts in the DESI Legacy Surveys and GAIA catalogs.

Key Results

Discovery: PSR J0125−5854 is a 24 ms pulsar with a dispersion measure (DM) of 11.66 pc cm $^{-3}$ , located at a high Galactic latitude of $-57.67^\circ$ .
Binary Nature: The pulsar is in a binary system with an orbital period exceeding 290 days. Refined timing analysis yields an orbital period of $P_b = 833.60 \pm 0.04$ days.
Orbital Parameters: The system has a projected semi-major axis of $x = 241.36 \pm 0.05$ light-seconds and a very low eccentricity of $e = 0.0052 \pm 0.0006$ .
Companion Mass: The minimum companion mass is determined to be $M_{c,min} = 0.4152 \pm 0.0001 M_\odot$ .
Spectral Properties: The pulsar exhibits a steep radio spectrum with a spectral index of $\alpha = -2.2 \pm 0.3$ . Flux densities range from $\sim17$ mJy at 154 MHz to $\sim0.18$ mJy at 1284 MHz.
Companion Identification: The orbital parameters and the low eccentricity strongly suggest the companion is a Helium white dwarf. An optical source ( $r=22.03$ ) consistent with a white dwarf at a distance of 0.5–1 kpc was identified in the DESI Legacy Surveys within the localization uncertainty region.
Distance: Depending on the electron density model used (NE2001, NE2025, or YMW16), the distance is estimated between 0.54 and 1.03 kpc.

Significance and Claims
The paper claims this discovery as the first MSP found with the MWA and the first pulsar confirmed from the deep-pass searches of the SMART survey. The detection of PSR J0125−5854 demonstrates the capability of low-frequency surveys to find MSPs in wide binary orbits, which are often missed by surveys with shorter integration times or those limited to the Galactic plane.

The authors highlight that the system's properties (long period, low eccentricity, He WD companion) align with the "wide" binary population, distinct from the tighter, more recycled systems. The discovery underscores the potential of the SMART survey to yield a significant population of MSPs as processing continues, providing a valuable reference for future low-frequency surveys planned with the SKA-Low. The paper notes that while the current data does not yet support a fully coherent timing solution (due to the large gap between observation epochs), the derived orbital parameters are robust enough to characterize the system's nature. The authors conclude that further observations are required to fully constrain the orbital and spin parameters and to confirm the coherent timing solution.