Deep Very Long Baseline Interferometry Constraints on Compact Radio Cores in Four Ultraluminous X-Ray Sources
Journal article, 2026
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 μJy. The corresponding 5σ flux density upper limits reach ∼26 μJy, implying radio luminosity limits LR ≲ 2 × 1033 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 nondetections 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.
Stellar mass black holes (1611)
Very long baseline interferometry (1769)
Ultraluminous x-ray sources (2164)
Radio jets (1347)
Author
Ailing Wang
Spallation Neutron Science Center
Chinese Academy of Sciences
Hua Feng
Chinese Academy of Sciences
T. An
Chinese Academy of Sciences
University of Science and Technology of China
Yijia Zhang
Tsinghua University
Jun Yang
Chalmers, Space, Earth and Environment, Onsala Space Observatory
Roberto Soria
The University of Sydney
Istituto nazionale di astrofisica (INAF)
Lian Tao
Chinese Academy of Sciences
Thomas D. Russell
Istituto nazionale di astrofisica (INAF)
Jing Guo
Tsinghua University
Liang Zhang
Chinese Academy of Sciences
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 1003 2Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
DOI
10.3847/1538-4357/ae63cb