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A NICER view of the millisecond pulsar PSR J2124$-$3358: evidence for a helium atmosphere

This study utilizes NICER and Chandra observations analyzed with the X-PSI package to model the millisecond pulsar PSR J2124$-$3358, finding that a helium atmospheric composition—likely resulting from the evaporation of a former binary companion—best fits the data and yields a mass of $1.8\pm0.5\,M_\odot$ and an equatorial radius of $11.7^{+2.6}_{-3.0}$ km.

Original authors: Denis González-Caniulef, Sebastien Guillot, Pierre Stammler, Lucien Mauviard, Christine Kazantsev, Anna L. Watts, Devarshi Choudhury, Bas Dorsman, Mariska Hoogkamer, Daniela Huppenkothen, Yves Kini, T

Published 2026-07-07

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CC BY 4.0

Original authors: Denis González-Caniulef, Sebastien Guillot, Pierre Stammler, Lucien Mauviard, Christine Kazantsev, Anna L. Watts, Devarshi Choudhury, Bas Dorsman, Mariska Hoogkamer, Daniela Huppenkothen, Yves Kini, Tuomo Salmi, Lucas Guillemot, Ismaël Cognard, Gilles Theureau

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

Imagine a neutron star as a cosmic lighthouse. It's a city-sized ball of matter so dense that a single teaspoon would weigh a billion tons. As it spins hundreds of times a second, it beams X-rays toward us like a searchlight. By studying the shape and brightness of these light beams, scientists can figure out how heavy the star is and how big it is. This is crucial because these stars are made of matter in a state we can't recreate in any lab on Earth; understanding them helps us understand the fundamental rules of physics.

The paper you provided is a detailed investigation of one specific lighthouse: a fast-spinning neutron star called PSR J2124−3358. Here is the story of what the scientists found, explained simply.

The Detective Work: Taking a Better Picture

For years, scientists have been trying to measure the size and weight of these stars, but it's like trying to measure a tiny, distant object through a foggy window. The "fog" here is the background noise of the universe and the limitations of our telescopes.

The team used two powerful space telescopes, NICER (which is like a high-speed camera on the International Space Station) and Chandra (a sharper, more precise camera), to take a closer look at this specific star. They gathered data over several years, filtering out the "static" and "glare" to get a clear picture of the star's pulse.

The Big Question: What is the Star Made Of?

The star's surface is covered in a thin layer of gas (an "atmosphere"). The scientists had to guess what this gas was made of to interpret the light correctly. They tested two main theories:

The Hydrogen Theory: The star is covered in a layer of hydrogen (the lightest element, like the fuel in a star).
The Helium Theory: The star is covered in a layer of helium (a heavier, inert gas).

The Analogy: Imagine trying to identify a person in a crowd by their silhouette. If you assume they are wearing a heavy winter coat (Hydrogen), you might guess they are shorter and stockier. If you assume they are wearing a light summer shirt (Helium), you might guess they are taller and leaner. The "coat" changes how you interpret the shape of the person.

The Findings: The Star Wears a "Helium Coat"

After running complex computer simulations (using a tool called X-PSI that acts like a cosmic 3D modeler), the team found that the Helium theory fit the data much better.

Why Helium? The star was once part of a binary system (a pair of stars orbiting each other). It likely "stole" material from its partner star to spin up to its current super-fast speed. Eventually, the partner star was evaporated by the intense energy of the pulsar. The scientists believe this process left a layer of helium on the surface, rather than hydrogen.
The Resulting Measurements: Because the "Helium coat" fits the data, the scientists calculated the star's properties based on this:
- Mass: It weighs about 1.8 times the mass of our Sun.
- Size: It has a radius of about 11.7 kilometers (roughly 7 miles).
- Note: The measurements have a wide margin of error (like saying "between 1.3 and 2.3 suns") because the star is very faint and the data is a bit "noisy."

If they had stuck with the Hydrogen theory, the star would have looked smaller and lighter (about 1.3 solar masses and 10.6 km radius), but the data didn't support that as well.

The Shape of the Light: Two Slightly Offset Spots

The X-rays don't come from the whole star; they come from two hot "spots" on the surface, like two glowing embers on a campfire.

The Helium Model: The two hot spots are almost directly opposite each other (like the North and South poles), suggesting the star's magnetic field is fairly straight and centered.
The Hydrogen Model: The spots would have to be in weird, offset positions to make the math work, suggesting a crooked magnetic field.

Since the Helium model fits the light pattern better, the scientists conclude the magnetic field is likely straight and centered.

The Challenge: A Faint Signal

The paper emphasizes that this star is very dim. It's like trying to hear a whisper in a noisy stadium. About 80% of the data the telescope collected was actually background noise (from the Sun, Earth's atmosphere, and other space sources), not the star itself. This is why the final measurements have large "uncertainty bars." The scientists did the best they could with the available data, but they need more observations to get a sharper picture.

Summary

In short, this paper is a successful attempt to weigh and measure a tiny, dense star by analyzing its X-ray light. The team concluded that the star is likely covered in helium, weighs about 1.8 suns, and is about 12 kilometers wide. This helps scientists build a better "rulebook" for how matter behaves under extreme pressure, even though they need more data to be 100% certain.

Technical Summary: A NICER view of the millisecond pulsar PSR J2124−3358: evidence for a helium atmosphere

Problem and Motivation
Determining the mass and radius of neutron stars (NSs) is critical for constraining the equation of state (EOS) of superdense matter, a regime inaccessible to terrestrial laboratories. While pulse profile modeling (PPM) using X-ray observations has successfully constrained the properties of several millisecond pulsars (MSPs), such as PSR J0030+0451 and PSR J0740+6620, analyses of additional sources are required to tighten EOS constraints. PSR J2124−3358, an isolated, rotation-powered MSP, represents a challenging target due to its faint X-ray emission and the dominance of background noise in available data. Previous preliminary analyses (e.g., S. Bogdanov et al. 2019b) utilized limited NICER data; this study aims to perform a comprehensive PPM analysis using extended NICER observations and archival Chandra data to derive robust mass and radius measurements and investigate the source's atmospheric composition.

Methodology
The authors employed the X-ray Pulse Simulation and Inference (X-PSI) package, a Bayesian framework utilizing relativistic ray-tracing to model X-ray pulse profiles. The analysis combined:

Data: $\sim$ 1.72 Ms of NICER observations (2017–2023) and three archival Chandra ACIS-S observations (2004, 2016). Chandra data were used to constrain the NICER background via joint fitting, leveraging Chandra's superior angular resolution to isolate the source from the surrounding pulsar wind nebula (PWN) and diffuse background.
Modeling: The study explored various hot spot configurations (Single-Temperature [ST], Concentric Double Temperature [CDT], and "Elsewhere" components representing the bulk surface) and two atmospheric compositions: fully ionized hydrogen and helium.
Inference: Bayesian parameter inference was performed using the PyMultiNest algorithm. Priors were set for mass ( $1.0-3.0 M_\odot$ ), radius, distance (based on EPTA parallax measurements), and atmospheric parameters. The analysis accounted for general relativistic effects (light bending, time delay) and special relativistic effects (Doppler shifts, aberration).

Key Results

Atmospheric Composition: The analysis favors a helium atmospheric composition over hydrogen. For the preferred ST+CDT model, the Bayesian evidence indicates a substantial (though not decisive) preference for helium ( $\Delta \log_{10} Z \approx 0.85$ ).
Mass and Radius:
- Helium Model: Yields a mass $M = 1.8 \pm 0.5 M_\odot$ and an equatorial radius $R_{eq} = 11.7^{+2.6}_{-3.0}$ km (medians and 68% credible intervals).
- Hydrogen Model: Results in lower values, with $M \approx 1.3 M_\odot$ and $R_{eq} \approx 10.6$ km. Notably, the hydrogen mass posterior peaks near the lower prior boundary ( $1.0 M_\odot$ ), suggesting the results may be truncated by the prior limits.
Hot Spot Geometry:
- Under the helium assumption, the preferred ST+CDT configuration shows hot spots with only slight deviations from antipodality, suggesting a magnetic field configuration close to a centered dipole.
- Under the hydrogen assumption, the model favors a configuration with a significant deviation from antipodality, implying an off-centered dipolar magnetic field.
Background Dominance: The study confirms that the NICER data for PSR J2124−3358 are background-dominated, with background contributions estimated at $\sim$ 77–82% of the total counts. The inclusion of Chandra data was essential for constraining this background.

Significance and Claims
The paper claims to provide substantial evidence for a helium atmosphere in an isolated MSP, a finding that challenges the assumption that MSPs universally possess hydrogen atmospheres. The authors suggest this helium composition may originate from the accretion and subsequent evaporation of a hydrogen-depleted binary companion (likely a helium white dwarf) during the pulsar's recycling phase.

The study emphasizes that while the derived mass and radius measurements are consistent with previous NICER results for other MSPs, the uncertainties remain large ( $\sim \pm 3$ km for radius, $\sim \pm 0.5 M_\odot$ for mass) due to the source's faintness, the background-dominated nature of the data, and the lack of an independent mass measurement (as the pulsar is isolated). Consequently, the authors state that these results do not yet provide tight enough constraints to significantly refine the EOS of superdense matter. They conclude that future multiwavelength studies, particularly those incorporating radio and gamma-ray pulse emission to constrain the magnetic geometry, are necessary to further confirm the atmospheric composition and improve the precision of physical parameter estimates.