Detection of a Radio Bubble around the Ultraluminous X-ray Source Holmberg IX X-1

TL;DR

This study uses radio observations to detect a bubble around the ULX Holmberg IX X-1, finding no core radio source, and suggests the bubble is likely inflated by winds and jets, similar to microquasar SS 433.

Contribution

First radio detection of a bubble around Holmberg IX X-1, constraining jet activity and proposing a wind+jet driven bubble model.

Findings

No unresolved radio core detected at the ULX position.

Radio bubble has a spectral slope of -0.56, similar to other ULXs.

Upper limit on core radio emission is 6.6 μJy.

Abstract

We present C and X-band radio observations of the famous utraluminous X-ray source (ULX) Holmberg IX X-1, previously discovered to be associated with an optical emission line nebula several hundred pc in extent. Our recent infrared study of the ULX suggested that a jet could be responsible for the infrared excess detected at the ULX position. The new radio observations, performed using the Karl G. Jansky Very Large Array (VLA) in B-configuration, reveal the presence of a radio counterpart to the nebula with a spectral slope of -0.56 similar to other ULXs. Importantly, we find no evidence for an unresolved radio source associated with the ULX itself, and we set an upper limit on any 5 GHz radio core emission of 6.6 $\mu$Jy ($4.1\times10^{32}$ erg s$^{-1}$). This is 20 times fainter than what we expect if the bubble is energized by a jet. If a jet exists its core component is unlikely to be responsible for the infrared excess unless it is variable. Strong winds which are expected in super-Eddington sources could also play an important role in inflating the radio bubble. We discuss possible interpretations of the radio/optical bubble and we prefer the jet+winds-blown bubble scenario similar to the microquasar SS 433.