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Astronomers used NuSTAR observations to separately study two low-mass X-ray binary systems near the Galactic Center, SLX 1744-299 and SLX 1744-300, for the first time in hard X-rays above 10 keV. Analysis of their X-ray luminosities and accretion rates suggests both systems likely have very short orbital periods under 90-155 minutes, with SLX 1744-299 showing particularly strong evidence of being an ultracompact X-ray binary where a neutron star pulls matter from a companion star in an extremely tight orbit.
Why it matters
Ultracompact X-ray binaries are rare systems that help scientists understand stellar evolution in extreme conditions and matter behavior under intense gravity. Identifying new members of this class improves our census of these systems in our galaxy and provides laboratories for studying accretion physics at low luminosities, a regime that remains poorly understood.
arXiv:2606.11133v1 Announce Type: new
Abstract: Persistent, low-luminosity low-mass X-ray binaries (LMXBs) offer a unique opportunity to study accretion in this poorly understood regime, as well as to unveil new members of the ultracompact X-ray binary (UCXB) family, characterised by orbital periods ($P_{rm orb}$) shorter than $sim 80$ min. We report on a NuSTAR archival observation that, for the first time above 10 keV, spatially resolves the Galactic Centre pair SLX 1744$-$299 and SLX 1744$-$300. We find SLX 1744$-$300 to be slightly brighter, with a flux ratio of $sim 1.15$, increasing to $sim 1.3$ when extrapolated to 0.5$-$10 keV. Both the timing (root-mean-square variability) and spectral properties (well described in both cases by a thermal Comptonisation model) indicate that the systems were in the hard state. The two sources, however, display markedly different behaviour throughout the observation. SLX 1744$-$299 shows a gradual flux decline consistent with a decrease in the mass-accretion rate, whereas SLX 1744$-$300 remains steady but exhibits two short-recurrence Type-I X-ray bursts indicative of mixed H/He burning. Combining our results with previously reported upper limits on the distance, we derive low persistent X-ray luminosities of $L_{rm X}lesssim 1.1times10^{36}$ erg s$^{-1}$ and $L_{rm X}lesssim 2.6times10^{36}$ erg s$^{-1}$ (3$-$78 keV) for SLX 1744$-$299 and SLX 1744$-$300, respectively. The corresponding mass-accretion rates, when compared with the critical values from the disc instability model, favour $P_{rm orb}lesssim 90$ min and $P_{rm orb}lesssim 105-155$ min. Although both limits are formally compatible with the UCXB regime, the case of SLX 1744$-$299 appears significantly more compelling, also considering the previously reported intermediate-duration burst.