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4U 1556-60 as a very faint neutron star X-ray binary at 700 pc with an undetected radio jet

Revisiting the low-mass X-ray binary 4U 1556-60 with a newly determined Gaia distance of ~700 pc reveals it to be a very faint, persistent ultracompact neutron star system with an unusually low X-ray luminosity and an undetected radio jet, distinguishing it from typical X-ray binaries and black hole candidates.

Original authors: Eliot C. Pattie, Thomas J. Maccarone, Thomas Russell, Matteo Bachetti, Nathalie Degenaar, Thomas Kupfer

Published 2026-02-17

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CC BY 4.0

Original authors: Eliot C. Pattie, Thomas J. Maccarone, Thomas Russell, Matteo Bachetti, Nathalie Degenaar, Thomas Kupfer

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 the universe as a vast, dark ocean. For decades, astronomers have been scanning this ocean with powerful flashlights, looking for "X-ray binaries"—cosmic couples where a tiny, dense star (like a neutron star or a black hole) is greedily eating material from a larger companion star.

One of these couples, named 4U 1556–60, was spotted over 50 years ago. But for a long time, we didn't really know who they were or how far away they lived. We thought they were a distant, massive couple living near the center of our galaxy, about 8,000 light-years away. Because we thought they were so far, we assumed they were incredibly bright and powerful, like a lighthouse seen from the other side of the ocean.

The Big Twist: They're Next Door!
Recently, a space telescope called Gaia took a new measurement, like using a high-precision ruler instead of a guess. It turned out 4U 1556–60 isn't a distant giant; it's a tiny neighbor living just 700 light-years away.

This discovery is like realizing a faint glow you thought was a distant campfire is actually a small candle sitting on your own porch. Because they are so close, their "brightness" (luminosity) is actually much weaker than we thought. They are one of the closest X-ray couples we know.

The Mystery of the Missing Radio Jet
When these cosmic couples eat, they often shoot out a powerful stream of particles, like a cosmic water hose. This is called a "radio jet."

Black Holes are like fire hoses: they shoot out powerful jets that are easy to see.
Neutron Stars are usually like garden hoses: they shoot weaker jets.

The authors of this paper used a giant radio telescope (the ATCA) to look for this "hose" from 4U 1556–60. They found nothing. Not even a drip. The jet is at least 1,000 times fainter than what we'd expect from a black hole. This is the smoking gun: 4U 1556–60 is definitely a neutron star, not a black hole. It's a very shy eater that barely shoots anything out.

The "Ultracompact" Dance
Because the couple is so close to us, and the companion star is so faint, the authors realized they must be dancing very tightly.

In a normal binary, the stars might be a few hours apart in their orbit.
In this system, the companion star is likely a "degenerate" star (a stellar corpse, like a white dwarf) that is so small and close that they orbit each other in less than an hour.

Think of it like two dancers holding hands so tightly they are practically spinning in place. This is called an Ultracompact X-ray Binary. It's a rare, high-speed dance.

Why Don't They Explode?
Neutron stars usually have a party trick: when they eat enough gas, they explode in a flash of light called a "Type I burst." It's like a pressure cooker letting off steam.

4U 1556–60 has never been seen to explode.
Why? Because they are eating so slowly (a "very faint" feast) that the pressure never builds up enough to trigger an explosion. It's like a drip-feed that never fills the pot. The authors calculate that if an explosion does happen, it might only occur once every few years or even decades.

The Hydrogen Puzzle
Usually, these tight-dancing couples have run out of hydrogen (the fuel for stars) and are made of heavier stuff like helium or carbon. But, 4U 1556–60 still shows traces of hydrogen in its light.

The Theory: The companion star might be an "evolved" star that hasn't been stripped of all its hydrogen yet. It's like a dancer who is still wearing their old costume even though the dance has changed. This makes 4U 1556–60 a unique, rare find.

What's Next?
This paper is a detective story that rewrites the history of a 50-year-old mystery.

Distance: They are close (700 light-years), not far.
Identity: They are a neutron star, not a black hole.
Nature: They are an "ultracompact" system, dancing in less than an hour.
Future: Because they are so close and spinning so fast, they might be a perfect target for future gravitational wave detectors (like LISA) to "hear" their dance through the fabric of space-time.

In short, 4U 1556–60 is a quiet, shy, ultra-fast cosmic couple living in our cosmic backyard, teaching us that even the faintest objects can hold the biggest secrets.

1. Problem Statement

4U 1556–60 is a persistent low-mass X-ray binary (LMXB) discovered in 1972. Historically, its distance was unknown and assumed to be near the Galactic Center (~8 kpc), implying a high X-ray luminosity ( $L_X \sim 5 \times 10^{36}$ erg s $^{-1}$ ) typical of active LMXBs. However, this distance assumption left several physical characteristics unexplained:

Accretor Nature: The lack of Type I X-ray bursts (common in neutron star LMXBs) and X-ray pulsations made the nature of the compact object (neutron star vs. black hole) ambiguous.
Spectral Properties: The source showed a hard spectral state without a clear thermal disk component, and optical spectra showed strong helium lines but variable hydrogen lines, complicating the classification of the donor star (e.g., ultracompact vs. standard LMXB).
Radio Jet: Previous radio observations were non-detections, but the sensitivity was insufficient to constrain the jet power relative to X-ray luminosity.

The recent Gaia Data Release 3 provided a parallax for the optical counterpart, suggesting a distance of only ~700 pc. This drastic reduction in distance (by an order of magnitude) fundamentally alters the source's intrinsic luminosity and requires a complete re-evaluation of its physical properties.

2. Methodology

The authors re-evaluated 4U 1556–60 using a multi-wavelength approach:

Distance Verification: They rigorously tested the Gaia parallax ( $1.4530 \pm 0.2644$ mas) against the possibility of an interloper star (unrelated foreground/background star) by analyzing proper motion consistency, infrared counterpart faintness, and local stellar density.
X-ray Observations: They obtained 75 ks of new data using NICER (Neutron Star Interior Composition ExploreR) in April–May 2023. They performed:
- Timing Analysis: Lomb-Scargle periodograms for orbital periods and power spectral density (PSD) analysis for spin periods (up to 1500 Hz).
- Spectral Analysis: Fitting the 0.5–8 keV spectrum with models including power laws, blackbodies ($bbodyrad$), and disk blackbodies ($diskbb$), including an iron line component.
Radio Observations: They conducted the first targeted radio observation in 45 years using the Australia Telescope Compact Array (ATCA) in November 2024. They observed at 5.5 GHz and 9.0 GHz to search for a radio jet, achieving deep sensitivity.
Comparative Analysis: They compared the derived X-ray and radio luminosities ( $L_X$ vs. $L_R$ ) against established correlations for Black Hole X-ray Binaries (BHXBs) and Neutron Star X-ray Binaries (NSXBs). They also utilized optical/infrared luminosity ratios and theoretical mass transfer rates to estimate the orbital period.

3. Key Contributions

Distance Confirmation: The paper confirms 4U 1556–60 is at ~700 pc, making it one of the closest known X-ray binaries and the closest known Roche-lobe overflowing LMXB.
Accretor Identification: The study provides the strongest evidence to date that the accretor is a neutron star, based on an extremely deep radio upper limit that deviates significantly from the BHXB jet correlation.
Ultracompact Classification: The authors propose 4U 1556–60 is an ultracompact X-ray binary (UCXB) with an orbital period likely between a few minutes and ~2 hours, driven by a degenerate or semi-degenerate donor.
Jet Physics: The non-detection of a radio jet provides a critical data point in the $L_R-L_X$ plane, suggesting that NSXBs at very low accretion rates may have suppressed jets due to magnetospheric effects, a phenomenon not seen in BHXBs.

4. Key Results

A. Distance and Counterpart

The Gaia counterpart is confirmed as the true optical source. The probability of a superimposed interloper (dwarf or white dwarf) is negligible ( $<10^{-5}$ ).
The distance is ~700 pc (geometric distance ~779 pc, photogeometric ~760 pc).

B. X-ray Properties

Luminosity: At 700 pc, the persistent X-ray luminosity is $L_X \approx 2 \times 10^{34}$ erg s $^{-1}$ (2–10 keV). This is unusually low for a persistent system but consistent with some UCXBs.
Spectrum: The spectrum is dominated by a power law (photon index $\Gamma \approx 1.53$ $Γ \approx 1.53$ ) with a small thermal component ( $kT \approx 0.9$ $k T \approx 0.9$ keV) and a weak iron line (EW $\approx 8$ $\approx 8$ eV).
- The thermal component implies a very small emitting radius ( $R \approx 0.21$ km), inconsistent with a standard accretion disk or black hole, but consistent with a hotspot on a neutron star or a boundary layer.
- $diskbb$ models were rejected as unphysical (inner radius < 1 km).
Timing:
- No Orbital Period: No periodicity was found in the 75 ks NICER data. This is attributed to a low inclination angle ( $i \lesssim 20^\circ$ ), inferred from the narrow width of the H $\alpha$ emission line.
- No Spin Period: No pulsations were detected (upper limit on pulse fraction $\sim 1\%$ ), suggesting either a weak magnetic field or aligned spin/magnetic axes.
- Variability: The source is remarkably stable (91% of data within 10% of median flux). The Power Spectral Density (PSD) shows no turnover down to $\sim 10^{-3}$ Hz, indicating a very low accretion rate.

C. Radio Jet

Non-detection: ATCA detected no radio counterpart.
Upper Limit: $L_R < 3 \times 10^{25}$ erg s $^{-1}$ (at 5 GHz).
Significance: This limit is $\sim 10^3$ times fainter than the expected radio luminosity for a black hole binary at the same X-ray luminosity. This strongly rules out a black hole accretor and confirms a neutron star. It also places the source in a regime where NSXB jets may be quenched by the magnetosphere at low accretion rates.

D. Binary Parameters and Evolution

Orbital Period: Based on the faintness of the optical/infrared counterpart and the high $L_X/L_{opt}$ ratio, the orbital period is estimated to be ultracompact (likely < 2 hours, possibly 20–60 minutes).
Donor Star: The presence of variable hydrogen lines in the optical spectrum is unusual for UCXBs (which typically have H-poor donors). The authors suggest the donor may be an evolved star that has not yet been fully stripped of hydrogen, or a white dwarf with trace hydrogen.
Bursting: The lack of Type I bursts is explained by the very low accretion rate ( $\dot{M} \sim 10^{-11} M_\odot$ /yr), which leads to long recurrence times (years) for thermonuclear ignition.

5. Significance

This paper fundamentally reclassifies 4U 1556–60 from a distant, standard LMXB to a nearby, very faint, ultracompact neutron star binary.

Astrophysical Insight: It provides a rare example of a persistent NSXB at a very low luminosity ( $10^{34}$ erg s $^{-1}$ ), challenging the understanding of disk ionization and mass transfer in UCXBs.
Jet Physics: The deep radio upper limit offers a new constraint on the $L_R-L_X$ correlation, suggesting that NS magnetospheres may effectively suppress jet production at low accretion rates, a mechanism not observed in black holes.
Gravitational Waves: At 700 pc, if the orbital period is sufficiently short (< 1 hour), 4U 1556–60 is a prime candidate for detection by the future LISA space-based gravitational wave observatory.
Future Work: The paper highlights the need for future observations to confirm the orbital period (via optical radial velocity or X-ray timing) to fully constrain the system's evolution and gravitational wave strain.