The NuSTAR view of Ultra-Compact X-ray Binaries
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arXiv
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| Natura: | Preprint |
| Pubblicazione: |
2026
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| author | Borghese, A. Padilla, M. Armas Muñoz-Darias, T. |
| author_facet | Borghese, A. Padilla, M. Armas Muñoz-Darias, T. |
| contents | Ultra-compact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries (LMXBs) characterised by tight orbits and hydrogen-poor donor stars. We present a spectral and timing study in the hard X-ray band of 11 of the 20 confirmed UCXBs, based on 37 archival NuSTAR observations. Using both X-ray colours and fractional root mean square values, we show that our sample spans the hard, soft, and intermediate X-ray states. Subsequently, we perform an X-ray spectral analysis using, when data allow it, the three-component model - an approach increasingly adopted for neutron star LMXBs. This work represents the largest LMXB sample analysed to date with this methodology. We focus on the properties of the X-ray continuum and report typical values for each X-ray state. Overall, UCXBs exhibit similar spectral properties to their longer-period counterparts, suggesting no major differences in the innermost regions of X-ray binaries, regardless of disc size or chemical composition. A possible exception is found in the soft-state sample, which shows Comptonisation fractions higher than those typically observed in regular LMXBs, although the statistics remain limited. Finally, we discuss the case of the slow X-ray pulsar 4U 1626-67, where we report the discovery of a very cold hard state with an electron temperature of ~6 keV - comparable to those usually observed in soft states of neutron-star LMXBs. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_13187 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | The NuSTAR view of Ultra-Compact X-ray Binaries Borghese, A. Padilla, M. Armas Muñoz-Darias, T. High Energy Astrophysical Phenomena Ultra-compact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries (LMXBs) characterised by tight orbits and hydrogen-poor donor stars. We present a spectral and timing study in the hard X-ray band of 11 of the 20 confirmed UCXBs, based on 37 archival NuSTAR observations. Using both X-ray colours and fractional root mean square values, we show that our sample spans the hard, soft, and intermediate X-ray states. Subsequently, we perform an X-ray spectral analysis using, when data allow it, the three-component model - an approach increasingly adopted for neutron star LMXBs. This work represents the largest LMXB sample analysed to date with this methodology. We focus on the properties of the X-ray continuum and report typical values for each X-ray state. Overall, UCXBs exhibit similar spectral properties to their longer-period counterparts, suggesting no major differences in the innermost regions of X-ray binaries, regardless of disc size or chemical composition. A possible exception is found in the soft-state sample, which shows Comptonisation fractions higher than those typically observed in regular LMXBs, although the statistics remain limited. Finally, we discuss the case of the slow X-ray pulsar 4U 1626-67, where we report the discovery of a very cold hard state with an electron temperature of ~6 keV - comparable to those usually observed in soft states of neutron-star LMXBs. |
| title | The NuSTAR view of Ultra-Compact X-ray Binaries |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2604.13187 |