NuSTAR Observations Resolve Two Galactic Center X-ray Binaries, Supporting Ultracompact Classification
Astronomers have used NuSTAR archival data to spatially separate the closely paired X-ray binary systems SLX 1744-299 and SLX 1744-300 above 10 keV for the first time, revealing distinct behaviours in each source. Both systems show low X-ray luminosities and spectral properties consistent with the hard accretion state, while SLX 1744-300 additionally displayed two Type-I X-ray bursts suggesting mixed hydrogen/helium burning. The findings strengthen the case that at least one of the pair — SLX 1744-299 — may be an ultracompact X-ray binary with an orbital period shorter than roughly 80 minutes.
Using a NuSTAR archival observation, researchers have for the first time resolved the Galactic Centre source pair SLX 1744-299 and SLX 1744-300 in the hard X-ray band above 10 keV, finding SLX 1744-300 to be slightly brighter with a flux ratio of approximately 1.15, rising to about 1.3 when extrapolated to the 0.5–10 keV band. Both sources were found to be in the hard accretion state, characterised by root-mean-square variability and spectra well described by thermal Comptonisation models. Despite this shared state, the two systems behaved differently during the observation: SLX 1744-299 exhibited a gradual flux decline consistent with a decreasing mass-accretion rate, while SLX 1744-300 remained steady but produced two short-recurrence Type-I X-ray bursts indicative of mixed H/He burning on the neutron star surface. Combining these results with previously reported distance upper limits, the team derived persistent X-ray luminosities of no more than ~1.1×10³⁶ erg/s and ~2.6×10³⁶ erg/s (3–78 keV) for SLX 1744-299 and SLX 1744-300, respectively. Comparing the inferred mass-accretion rates against predictions from the disc instability model favours orbital periods shorter than ~90 minutes for SLX 1744-299 and shorter than ~105–155 minutes for SLX 1744-300, placing both formally within or near the ultracompact X-ray binary regime, with SLX 1744-299 presenting the more compelling case, further supported by a previously reported intermediate-duration X-ray burst.
What's missing
The orbital periods of both systems have not been directly measured; all period constraints are indirect, derived from accretion rate comparisons with the disc instability model and distance upper limits rather than from timing of eclipses or pulsations. The nature of the donor stars (e.g., helium white dwarf, hydrogen-depleted star) remains unconfirmed, and the distance upper limits themselves carry uncertainties that propagate into the luminosity and accretion-rate estimates. Direct confirmation of the ultracompact classification would require detection of the orbital period or spectroscopic identification of the donor.
What different sources said
- arXiv astro-phCenter
Resolving SLX 1744-299 and SLX 1744-300 in the hard X-ray band: implications for their ultracompact nature
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