Deep VLBI constraints on compact radio cores in four ultraluminous X-ray sources

TL;DR

This study used high-sensitivity VLBI observations to search for compact radio cores in four ULXs, setting upper limits that challenge the presence of persistent bright cores and suggesting variability and absorption effects.

Contribution

First high-sensitivity VLBI constraints on ULX radio cores, revealing non-detections and variability, informing models of ULX emission mechanisms.

Findings

No compact emission detected at 5-20 μJy levels.

Radio luminosity limits are below 2 x 10^{33} erg s^{-1}.

Holmberg II X-1's core shows long-term variability.

Abstract

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 $\mu$Jy. The corresponding $5\sigma$ flux density upper limits reach $\sim 26\,\mu\mathrm{Jy}$, 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.