PSR J0653+0443: An Eclipsing Millisecond Pulsar in an Extremely Eccentric Double Neutron Star System

Z. L. Yang; J. L. Han; L. G. Hou; Z. W. Chen; Y. G. Gong; Y. L. Guo; Y. J. Lu; C. Zhang; K. Y. Song; B. Liu; D. Lai; C. Wang; W. Q. Su; P. F. Wang; X. P. You; N. N. Cai; Q. Gao; W. C. Jing; B. Wang; H. G. Wang; T. Wang; J. Xu; Y. Yan; D. J. Zhou

AI summary · arXiv: 2610.29838v1 · astro-ph.HE

PSR J0653+0443: An Eclipsing Millisecond Pulsar in an Extremely Eccentric Double Neutron Star System

Z. L. Yang, J. L. Han, L. G. Hou, Z. W. Chen, Y. G. Gong, Y. L. Guo, Y. J. Lu, C. Zhang,

and 16 more authors

K. Y. Song, B. Liu, D. Lai, C. Wang, W. Q. Su, P. F. Wang, X. P. You, N. N. Cai, Q. Gao, W. C. Jing, B. Wang, H. G. Wang, T. Wang, J. Xu, Y. Yan, D. J. Zhou

Original paper Submitted to arXiv on 21 Oct. 2026 31 pages, 14 figures, original submitted version, accepted by Nature physics License: CC BY-NC-SA 4.0 Also in: fr

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The essentials

The paper reports PSR J0653+0443, an eclipsing double neutron star with record-high eccentricity, unusual timing effects, and a rare formation pathway.

Question

What are the orbital and relativistic properties of PSR J0653+0443, and what do its extreme orbit and gravitational-wave emission imply for binary evolution and future detection?

Method

The authors used FAST observations from 2022–2025, including a 2-hour observation near periastron, and analyzed 342 pulse times of arrival spanning more than two years. They fitted a phase-connected timing solution using modified high-eccentricity pulsar timing models, then used Bayesian and chi-squared analyses to infer system parameters; they also modeled binary evolution and population synthesis and calculated the system’s gravitational-wave signal.

Findings

Result The system has an orbital eccentricity of 0.99238692 and an orbital period of 79.08406184 days, making it the most eccentric stellar binary reported by the authors.
See the passage in the paper
We conducted a 2-hour tracking observation of PSR J0653+0443 covering the periastron passage (see zoomed part of Fig. 1), and precisely measured the orbital eccentricity ($e=0.99238692\pm 0.00000005$), the highest ever measured for stellar binary systems.
Result FAST observations detected an 85-second radio eclipse near conjunction, consistent with the system having a nearly edge-on orbit.
See the passage in the paper
We also detected an 85-second eclipse of the radio emission, occurring near $\theta=90^{\circ}-\omega=46^{\circ}$ (grey-shaded region in Fig. 2), when the pulsar, its companion, and the barycenter were aligned along the line of sight (Fig. 1).
Result Orbital-phase-dependent periastron advance produces a calculated pulse-arrival-time modulation from −2.91 to +2.49 ms, over an order of magnitude larger than in the next-largest comparison system.
See the passage in the paper
As demonstrated in Fig. 3D, $\Delta_{\rm RB,parv}$ varies in the range of $-2.91$ ms to +2.49 ms. Compared to the other DNS systems (see Table S3), it is more than one order of magnitude greater than the next largest one ($-159$ µs to 128 µs for PSR J2140$-$2311B Balakrishnan et al. (2023)).
Result Population synthesis estimates that systems with eccentricity above 0.95 and orbital periods of 50–110 days make up at most about 5×10⁻³ of surviving DNS systems with observable pulsars.
See the passage in the paper
For DNS systems with a larger parameter space, such as $e>0.95$ and $P_{\rm orb}\in[50,110]$ days, the fraction is only about $\lesssim 5\times 10^{-3}$.
Authors’ interpretation The authors conclude that a large kick during the second neutron star’s iron-core-collapse supernova is needed to explain the extreme eccentricity, making the discovery a challenge for current binary-evolution understanding.
See the passage in the paper
The fact that we detected such a binary with such an extreme eccentricity among 28 Galactic field DNS systems (Table S3) suggesting that either we are very lucky or the current binary evolution theory and current understanding of core-collapse SNe needs modifications.

Limitations

The Einstein delay cannot currently be measured directly because it is strongly covariant with an orbital timing parameter.
See the passage in the paper
Currently, $\gamma$ can not be directly measured, we treated it as a fixed parameter when $\gamma$ is a parameter of the timing model (see Table S5).
The authors state that several other post-Keplerian effects are too small to detect and therefore are not included in the timing model.
See the passage in the paper
Besides the post-Keplerian parameter $\dot{\omega}$, $r$, $s$ and $\gamma$, other post-Keplerian parameters, including the orbital period derivative $\dot{P}_{\rm orb}$, the periodic relativistic corrections to the orbital motion $\delta_{\rm r}$ and $\delta_{\theta}$, and the two aberration parameters $A$ and $B$ Damour and Deruelle (1986), are currently too small to be detected and are therefore not yet required in the timing model.
The predicted gravitational-wave signal is below the sensitivity of LISA, Taiji, and TianQin, with detectability discussed only for future detector concepts.

Why it matters

The discovery provides an unusually eccentric Galactic-field double neutron star for studying relativistic timing and gravitational-wave emission from highly eccentric binaries. The authors argue that its rarity challenges current models of binary evolution and core-collapse supernovae, while its predicted signal could test high-harmonic gravitational-wave waveforms with future detector concepts.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: double neutron star; pulsar timing; extreme eccentricity; supernova kick; gravitational waves

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Comprehensive Summary
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Original abstract

The paper reports the discovery and analysis of PSR J0653+0443, a Galactic field double neutron star system. It examines how the system’s unusually eccentric orbit affects pulsar timing, binary evolution, and gravitational-wave emission. The authors observed the 47.5-millisecond pulsar with the FAST radio telescope from 2022 to 2025. They collected data across the orbit, including during a two-hour session at periastron, and used them to measure pulse arrival times and derive a phase-connected timing solution. Because conventional orbital parameters become strongly correlated at such extreme eccentricity, the authors introduced a revised timing parameterization to improve the fit. They also modeled relativistic timing effects, investigated possible formation pathways using stellar-evolution and population-synthesis calculations, and estimated the system’s gravitational-wave signal. The system has an orbital period of 79.084 days and an eccentricity of 0.99238692, the highest reported for a stellar binary in the paper. Although the orbit is wide on average, the neutron stars approach to about 0.83 solar radii at periastron. Their relative speed reaches roughly 1,140 kilometres per second, and their acceleration reaches about 1,110 metres per second squared. The pulsar traverses true anomaly from −90 to +90 degrees in only about 46 minutes. Timing measurements yield a total system mass of 2.814 solar masses and estimated individual masses of 1.38 and 1.44 solar masses. The inferred orbital inclination is 76 degrees, although it remains substantially uncertain. An 85-second radio eclipse near conjunction, when the stars are separated by about 2.28 light-seconds, independently supports a nearly edge-on geometry. The authors attribute the eclipse to the companion’s magnetosphere and estimate its effective eclipsing size at roughly 40,000 kilometres. The timing analysis measures a secular periastron advance of 0.017791 degrees per year, consistent with general relativity. The orbit’s rapid, phase-dependent periastron motion produces a predicted arrival-time modulation ranging from about −2.91 to +2.49 milliseconds—more than ten times the largest comparison value cited for another double neutron star. The estimated non-relativistic contribution to periastron advance is −1.5 × 10⁻⁴ degrees per year, but it is consistent with zero at 95% confidence. Shapiro-delay parameters are only marginally detected. The Einstein delay is estimated at 45 ± 4 milliseconds, but it cannot yet be measured independently because it is strongly covariant with an orbital parameter. The authors argue that the system’s extreme eccentricity most likely resulted from a large kick to the second-born neutron star during an iron-core-collapse supernova. Their illustrative progenitor model consists of a neutron star and a helium star in an orbit of about 0.9 days before the explosion. They infer a likely kick velocity of roughly 200–800 kilometres per second. Population-synthesis results indicate that systems with eccentricities above 0.95 and orbital periods of 50–110 days are rare, accounting for no more than about 0.5% of observable surviving double neutron stars. This rarity raises questions about supernova kicks and binary evolution, although the estimate depends on model assumptions and kick distributions. The system emits gravitational-wave bursts concentrated near periastron. Assuming a distance of 1 kiloparsec, the energy-flux peak occurs at 0.357 millihertz, near the 2,440th orbital harmonic. The signal falls below the stated sensitivities of LISA, Taiji, and TianQin.

A Double Neutron Star System with an Extreme Orbit

Most double neutron star systems follow orbits that are close to circular. PSR J0653+0443 is a striking exception: its orbit is so elongated that the two stars swing from a wide separation to a close, fast encounter every 79 days. In “PSR J0653+0443: An Eclipsing Millisecond Pulsar in an Extremely Eccentric Double Neutron Star System,” Z. L. Yang and colleagues report the discovery and analysis of this rare Galactic system—and explore what it reveals about pulsar timing, stellar evolution, and gravitational waves.

The system’s pulsar spins every 47.5 milliseconds. Researchers observed it with China’s Five-hundred-meter Aperture Spherical Telescope, or FAST, from 2022 to 2025, collecting measurements across its orbit. One particularly valuable observing session lasted two hours around periastron, the point of closest approach. By tracking when pulses arrived, the team built a phase-connected timing solution: a model that follows each pulse consistently over the full observing span.

From a Wide Orbit to a Brief, Fierce Encounter

PSR J0653+0443 takes 79.084 days to complete an orbit, but its eccentricity is an extraordinary 0.99238692—the highest reported for a stellar binary in the paper. The stars are far apart for much of the orbit, yet approach to about 0.83 times the Sun’s radius at periastron. There, their relative speed reaches roughly 1,140 kilometres per second, and their acceleration is about 1,110 metres per second squared.

The close encounter is remarkably brief. The pulsar moves through the central portion of its orbit, from true anomaly −90 to +90 degrees, in only about 46 minutes. That rapid change makes the system both a challenge for timing and a natural laboratory for studying strong, rapidly changing orbital motion.

At such extreme eccentricity, standard ways of describing an orbit can make different parameters strongly entangled in the timing analysis. The researchers therefore introduced a revised parameterization to improve the fit. Their measurements yield a total system mass of 2.814 solar masses, with estimated individual masses of 1.38 and 1.44 solar masses.

Relativity, Timing Effects, and a Radio Eclipse

The timing data show that the orbit’s periastron advances by 0.017791 degrees per year, consistent with the prediction of general relativity. The eccentric orbit also creates a large, phase-dependent effect on pulse arrival times: the predicted modulation ranges from about −2.91 to +2.49 milliseconds. That is more than ten times the largest comparison value cited for another double neutron star system in the paper.

Other relativistic effects remain harder to pin down. The estimated Einstein delay is 45 ± 4 milliseconds, but it cannot yet be measured independently because it is strongly correlated with an orbital parameter. Shapiro-delay parameters are only marginally detected. The estimated non-relativistic contribution to periastron advance is −1.5 × 10⁻⁴ degrees per year, consistent with zero at 95% confidence.

The system also provides an independent clue to its orientation. The inferred orbital inclination is about 76 degrees, though with substantial uncertainty. Near conjunction, the pulsar’s radio signal disappears for approximately 85 seconds. At that point the stars are separated by about 2.28 light-seconds. The authors attribute the eclipse to the companion neutron star’s magnetosphere and estimate an effective eclipsing size of roughly 40,000 kilometres. The eclipse supports a nearly edge-on orbit.

How Did the Orbit Become So Extreme?

The authors investigate the system’s history using stellar-evolution and population-synthesis calculations. Their preferred explanation is a large kick delivered to the second-born neutron star during an iron-core-collapse supernova. As an illustrative pathway, they consider a neutron star paired with a helium star in an orbit of about 0.9 days before the explosion. The resulting kick is likely to have been roughly 200–800 kilometres per second.

Such systems appear to be uncommon. In the population-synthesis results, double neutron stars with eccentricities above 0.95 and orbital periods between 50 and 110 days make up no more than about 0.5% of observable surviving systems. That estimate depends on the models and assumed kick distributions, but the rarity highlights open questions about supernova explosions and the evolution of massive binary stars.

A Gravitational-Wave Signal Beyond Current Sensitivity

Rather than emitting gravitational waves uniformly around its orbit, PSR J0653+0443 produces bursts concentrated near periastron, when the stars move fastest and are closest together. Assuming a distance of 1 kiloparsec, the energy-flux peak is predicted at 0.357 millihertz, near the 2,440th orbital harmonic.

Even with this distinctive high-harmonic signal, the estimated emission falls below the stated sensitivities of LISA, Taiji, and TianQin. The system is therefore not expected to be detectable by those observatories under the paper’s assumptions. Its value is nevertheless considerable: precise radio timing lets researchers probe a rare, rapidly changing neutron-star orbit and test ideas about how supernova kicks shape the lives of compact binaries.

Created on 31 Oct. 2026

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PSR J0653+0443: An Eclipsing Millisecond Pulsar in an Extremely Eccentric Double Neutron Star System

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

A Double Neutron Star System with an Extreme Orbit

From a Wide Orbit to a Brief, Fierce Encounter

Relativity, Timing Effects, and a Radio Eclipse

How Did the Orbit Become So Extreme?

A Gravitational-Wave Signal Beyond Current Sensitivity

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