Deep VLBI constraints on compact radio cores in four ultraluminous X-ray sources
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arXiv
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| Auteurs principaux: | , , , , , , , , , |
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| Format: | Preprint |
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2026
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| _version_ | 1866918462800003072 |
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| author | Wang, Ailing Feng, Hua An, Tao Zhang, Yijia Yang, Jun Soria, Roberto Tao, Lian Russell, Thomas Guo, Jing Zhang, Liang |
| author_facet | Wang, Ailing Feng, Hua An, Tao Zhang, Yijia Yang, Jun Soria, Roberto Tao, Lian Russell, Thomas Guo, Jing Zhang, Liang |
| contents | We present high-sensitivity Very Long Baseline Interferometry (VLBI) observations of four ultraluminous X-ray sources (ULXs): Holmberg II X-1, IC 342 X-1, NGC 6946 X-1, and NGC 925 X-1. No compact emission was detected on milliarcsecond scales, with rms noise levels reaching approximately 5--20 $μ$Jy. The corresponding $5σ$ flux density upper limits reach $\sim 26\,μ\mathrm{Jy}$, implying radio luminosity limits $L_{\rm R} \lesssim 2 \times 10^{33}\,\mathrm{erg\,s^{-1}}$. This disfavors any persistently bright hard-state-like compact core at our sensitivity level. The previously reported VLBI core in Holmberg II X-1 exhibits significant long-term variability, broadly consistent with an overall decline over the past decades. This behavior is consistent with emission from optically-thin ejecta undergoing adiabatic expansion. The VLBI non-detections may reflect intrinsically weak/intermittent compact emission, and/or low--surface--brightness structure that is resolved out by VLBI, and/or absorption/propagation effects such as free--free absorption in dense, ionized winds. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_20609 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Deep VLBI constraints on compact radio cores in four ultraluminous X-ray sources Wang, Ailing Feng, Hua An, Tao Zhang, Yijia Yang, Jun Soria, Roberto Tao, Lian Russell, Thomas Guo, Jing Zhang, Liang High Energy Astrophysical Phenomena We present high-sensitivity Very Long Baseline Interferometry (VLBI) observations of four ultraluminous X-ray sources (ULXs): Holmberg II X-1, IC 342 X-1, NGC 6946 X-1, and NGC 925 X-1. No compact emission was detected on milliarcsecond scales, with rms noise levels reaching approximately 5--20 $μ$Jy. The corresponding $5σ$ flux density upper limits reach $\sim 26\,μ\mathrm{Jy}$, implying radio luminosity limits $L_{\rm R} \lesssim 2 \times 10^{33}\,\mathrm{erg\,s^{-1}}$. This disfavors any persistently bright hard-state-like compact core at our sensitivity level. The previously reported VLBI core in Holmberg II X-1 exhibits significant long-term variability, broadly consistent with an overall decline over the past decades. This behavior is consistent with emission from optically-thin ejecta undergoing adiabatic expansion. The VLBI non-detections may reflect intrinsically weak/intermittent compact emission, and/or low--surface--brightness structure that is resolved out by VLBI, and/or absorption/propagation effects such as free--free absorption in dense, ionized winds. |
| title | Deep VLBI constraints on compact radio cores in four ultraluminous X-ray sources |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2604.20609 |