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Resolving SLX 1744-299 and SLX 1744-300 in the hard X-ray band: implications for their ultracompact nature

This study utilizes NuSTAR observations to spatially resolve the Galactic Centre pair SLX 1744-299 and SLX 1744-300 in the hard X-ray band, revealing their hard-state properties and low luminosities which suggest orbital periods consistent with the ultracompact X-ray binary regime, particularly for SLX 1744-299.

Original authors: Enzo A. Saavedra, Montserrat Armas Padilla, Teo Muñoz-Darias

Published 2026-06-10

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CC BY 4.0

Original authors: Enzo A. Saavedra, Montserrat Armas Padilla, Teo Muñoz-Darias

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 center of our galaxy as a busy, crowded cosmic neighborhood. For decades, astronomers looked at a specific spot there and saw what they thought was a single, dim star system. But in reality, it was two distinct systems, SLX 1744−299 and SLX 1744−300, standing so close together in the sky (about the width of a human hair held at arm's length) that older telescopes couldn't tell them apart. They were like two fireflies blinking in the same spot, merging into one blurry glow.

This new study uses a powerful space telescope called NuSTAR, which acts like a high-resolution camera for "hard" X-rays (energetic light that passes through things softer X-rays can't). For the first time, NuSTAR could separate these two fireflies and look at them individually.

Here is what the astronomers found, explained simply:

1. The "Hard" State: A Cosmic Furnace

Both systems are Low-Mass X-ray Binaries (LMXBs). Think of them as a cosmic dance where a small, dense star (a neutron star) is stealing gas from a smaller, lighter companion star. As this gas spirals down, it heats up and glows brightly in X-rays.

The study found that both systems were in what astronomers call the "hard state."

The Analogy: Imagine a campfire. In a "soft" state, the fire is a gentle, warm, orange glow (mostly low-energy light). In the "hard" state, the fire is roaring with intense, blue-white heat and sparks (high-energy X-rays).
The Finding: Both systems were roaring with this high-energy "hard" light. The data showed they were churning out energy in a way that fits perfectly with a neutron star eating gas at a steady, intense pace.

2. Two Different Personalities

Even though they are neighbors and look similar, the two systems behaved very differently during the observation:

SLX 1744−299 (The Fading Star): This system was slowly dimming over the course of the observation.
- The Analogy: Imagine a candle that is slowly running out of wax. As the fuel (gas) supply dwindles, the flame gets smaller, and the light changes color.
- The Science: The astronomers saw the gas falling onto the neutron star slowing down. As the "fuel" decreased, the fire got hotter and harder, but the total amount of light dropped. This suggests the star is running out of gas to eat.
SLX 1744−300 (The Steady Eater with Burps): This system stayed steady in brightness, but it did something surprising: it had two sudden, short "bursts" of light.
- The Analogy: Imagine a person eating a steady meal but suddenly having two quick, intense hiccups or burps.
- The Science: These were Type-I X-ray bursts. They happen when the gas (a mix of hydrogen and helium) piled up on the neutron star's surface and suddenly ignited in a nuclear explosion, like a tiny, controlled supernova. The fact that they happened so quickly (about 30 seconds each) tells us the fuel being burned is a mix of hydrogen and helium.

3. Are They "Ultracompact"?

A major goal of the study was to see if these systems are Ultracompact X-ray Binaries (UCXBs).

The Definition: UCXBs are systems where the two stars are so close together that they orbit each other in less than 80 minutes. It's like a dance where the partners are spinning so fast they are practically touching.
The Clue: Because these systems are so faint (low luminosity), they should be UCXBs. If they were normal, wider systems, they would likely flicker on and off (transient) rather than shining steadily. The fact that they shine steadily at such low brightness suggests their "dance floor" (the gas disk) is tiny and stable.
The Verdict:
- SLX 1744−299: The evidence is very strong. Its low brightness and the type of bursts it has previously shown make it a very likely candidate for being an ultracompact system (orbiting in under 90 minutes).
- SLX 1744−300: It is possible it is ultracompact, but the evidence is weaker. It could be a slightly wider system, or it could be one, but we aren't 100% sure yet.

Summary

This paper is like finally putting on a pair of high-tech glasses to look at a crowded street corner. We realized there were two distinct people there, not one.

One person (SLX 1744−299) is slowly fading away as their fuel runs out, and they are almost certainly a very tight, fast-spinning pair of stars.
The other person (SLX 1744−300) is steady but has sudden, fiery burps of energy. They might be a tight pair too, but we need more clues to be sure.

The study confirms that these two systems are unique laboratories for understanding how stars eat gas in extreme, hydrogen-poor environments, and it gives us the best look yet at how they behave when they are "hard" and energetic.

Technical Summary: Resolving SLX 1744−299 and SLX 1744−300 in the Hard X-ray Band

Problem and Motivation
Persistent, low-luminosity low-mass X-ray binaries (LMXBs) represent a poorly understood regime of accretion physics. These systems are of particular interest as potential members of the ultracompact X-ray binary (UCXB) family, characterized by orbital periods ( $P_{\text{orb}}$ ) shorter than $\sim80$ minutes. Identifying UCXBs is observationally challenging due to the difficulty in measuring orbital periods directly, especially when optical counterparts are faint or absent. Indirect diagnostics, such as burst properties and persistent luminosity levels, are often required.

The Galactic Centre pair SLX 1744−299 and SLX 1744−300 presents a specific challenge: they are separated by only $\sim2.7$ arcminutes. Historically, this small angular separation prevented the resolution of their individual contributions in hard X-rays, leading to ambiguous interpretations of their variability and spectral properties. While previous studies identified SLX 1744−299 as a UCXB candidate based on intermediate-duration bursts and low luminosity, the nature of SLX 1744−300 remained largely unconstrained.

Methodology
The authors analyzed an archival NuSTAR observation (ObsID 30401036002) of the field, conducted on July 29, 2018. The observation provided a unique opportunity to spatially resolve the two sources for the first time above 10 keV, utilizing NuSTAR's broad energy coverage (3–78 keV) and superior spatial resolution.

Data Reduction: Data were processed using NuSTARDAS and HEASOFT v6.34. Source events were extracted from 50-arcsecond radius regions, while background events were taken from source-free regions on the same chip. Numerical simulations using a King profile confirmed that cross-contamination between the two sources was negligible ( $<1\%$ ).
Temporal Analysis: Power density spectra (PDS) were generated to search for pulsations and calculate fractional root-mean-square (rms) variability in the 0.1–64 Hz range.
Spectral Analysis: Time-averaged and time-resolved spectra were fitted using xspec. Models included interstellar absorption (TBabs) and thermal Comptonisation (nthComp) with seed photons from either a blackbody (BB) or a multicolour disc (MCD). The analysis accounted for NuSTAR calibration uncertainties by allowing gain offsets to vary.
Burst Analysis: Two Type-I X-ray bursts detected in SLX 1744−300 were modeled using a linear rise and exponential decay profile to determine recurrence times and spectral evolution.

Key Results

Spatial Resolution and Flux Ratio:
The observation successfully resolved the two sources. SLX 1744−300 was found to be slightly brighter than SLX 1744−299 in the 3–78 keV band, with a flux ratio of $\sim1.15$ . When extrapolated to the 0.5–10 keV band, this ratio increases to $\sim1.3$ . This contrasts with previous soft X-ray measurements where SLX 1744−299 appeared brighter, suggesting the flux ratio is variable across timescales.
Accretion State:
Both sources exhibited high fractional rms variability ( $\sim21–23\%$ ) and spectra well-described by a thermal Comptonisation model. These properties indicate that both systems were in the hard state during the observation.
- SLX 1744−299: Showed a photon index $\Gamma \approx 2.33$ and an electron temperature $kT_e \approx 38$ keV.
- SLX 1744−300: Showed a photon index $\Gamma \approx 2.28$ and a lower electron temperature $kT_e \approx 8.7$ keV.
  In both cases, adding a soft thermal component (BB or MCD) did not significantly improve the fit, and any such contribution was minimal ( $<2\%$ of the total flux).
Temporal Evolution:
The two sources displayed markedly different behaviors over the $\sim79$ ks observation:
- SLX 1744−299: Exhibited a gradual flux decline of $\sim28\%$ . Time-resolved spectroscopy revealed a hardening spectrum (decreasing $\Gamma$ , decreasing seed photon temperature, and increasing electron temperature), consistent with a decreasing mass-accretion rate.
- SLX 1744−300: Remained steady in flux but exhibited two short Type-I bursts (duration $\sim30$ s). Spectral analysis of the bursts showed cooling temperatures and increasing emission radii, consistent with the cooling of a neutron star photosphere. The recurrence time ( $\tau_{\text{rec}} \lesssim 5$ h) and burst properties suggest mixed hydrogen/helium burning.
Luminosity and Orbital Period Constraints:
Using previously reported upper limits on distance ( $7.2 \pm 1.4$ kpc for SLX 1744−299 and $10.3 \pm 0.8$ kpc for SLX 1744−300), the authors derived persistent X-ray luminosities of $L_X \lesssim 1.1 \times 10^{36}$ erg s $^{-1}$ and $L_X \lesssim 2.6 \times 10^{36}$ erg s $^{-1}$ (3–78 keV), respectively.
Comparing the inferred mass-accretion rates with the critical stability thresholds from the Disc Instability Model (DIM), the authors derived upper limits on the orbital periods:
- SLX 1744−299: $P_{\text{orb}} \lesssim 90$ min.
- SLX 1744−300: $P_{\text{orb}} \lesssim 105–155$ min.

Significance and Claims
The paper claims that the NuSTAR observation provides the first hard X-ray resolution of the SLX 1744 pair, allowing for a detailed characterization of their individual accretion states.

UCXB Candidates: The derived orbital period limits are formally compatible with the UCXB regime ( $P_{\text{orb}} \lesssim 80$ min). However, the authors state that the case for SLX 1744−299 is "significantly more compelling" than for SLX 1744−300. This conclusion is based on SLX 1744−299's stricter orbital period limit ( $\lesssim 90$ min), its lower luminosity, and the presence of previously reported intermediate-duration bursts.
Accretion Physics: The study highlights that SLX 1744−299's spectral evolution is consistent with a declining mass-accretion rate, while SLX 1744−300's stability and burst activity suggest a different accretion regime, potentially involving mixed H/He fuel.
Model Consistency: The spectral properties (dominance of Comptonisation, lack of significant soft thermal components) are consistent with theoretical expectations for low-luminosity hard-state NS-LMXBs, where the accretion disc may remain fully ionised at low rates, allowing for persistent emission.

The authors conclude that while both systems are candidates for the UCXB family, SLX 1744−299 remains the stronger candidate based on the combined evidence of low luminosity, stability constraints, and burst characteristics.

1. The "Hard" State: A Cosmic Furnace

2. Two Different Personalities

3. Are They "Ultracompact"?

Summary

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