NuSTAR Observations Resolve Two Galactic Center X-ray Binaries, Supporting Ultracompact Classification
Astronomers using NuSTAR have for the first time spatially resolved SLX 1744-299 and SLX 1744-300 above 10 keV, two low-luminosity X-ray binaries in the Galactic Center. Both systems display properties consistent with ultracompact X-ray binaries (UCXBs), characterized by orbital periods shorter than ~80 minutes, though SLX 1744-299 presents a more compelling case. This work advances understanding of accretion in the poorly-studied low-luminosity regime and helps identify new UCXB family members.
Researchers analyzed archival NuSTAR observations of SLX 1744-299 and SLX 1744-300, a pair of low-mass X-ray binaries near the Galactic Center, achieving the first hard X-ray spatial resolution of these sources above 10 keV. SLX 1744-300 was found to be slightly brighter with a flux ratio of ~1.15, while both systems exhibited thermal Comptonisation spectral properties indicating a hard accretion state. The two sources showed markedly different behavior: SLX 1744-299 displayed a gradual flux decline consistent with decreasing mass-accretion rate, whereas SLX 1744-300 remained steady but produced two Type-I X-ray bursts indicative of mixed hydrogen/helium burning. Derived persistent X-ray luminosities and mass-accretion rates, when compared with disc instability model predictions, favor orbital periods placing both systems in or near the ultracompact regime, with SLX 1744-299 presenting the stronger case for UCXB classification.
What's missing
The study does not discuss potential observational biases in detecting UCXBs or how selection effects might affect the completeness of known UCXB populations. Additionally, the paper does not address implications for compact object merger rates or gravitational wave sources, which are scientifically relevant given UCXB evolutionary pathways.
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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