# Observational limits on the X-ray emission from the bubble nebula surrounding Ho IX X-1

## TL;DR

This study used archival Chandra data to search for X-ray emission from the bubble nebula around ULX Holmberg IX X-1, finding no detectable X-ray bubble down to a luminosity limit, and discussing implications for future observations.

## Contribution

First detailed search for X-ray bubble emission around Holmberg IX X-1, setting upper limits and highlighting the potential of future X-ray missions like Athena.

## Key findings

- No strong evidence for X-ray bubble emission at luminosities above 2×10^{36} erg s^{-1}.
- The nebular emission does not significantly affect the ULX's X-ray spectrum.
- Future missions could detect such X-ray nebulae, constraining outflow kinematics.

## Abstract

Optical and radio observations of shock-ionised bubble nebulae surrounding ultraluminous X-ray sources (ULXs) suggest that they are powered by jets or super-critical outflows presumably launched from the ULX accretion disc. Recent simulations of these systems have shown that the shocked wind can emit thermal X-rays with estimated luminosities $\sim 10^{36}$ erg s$^{-1}$. In this work, we investigated whether it is possible to detect and spatially resolve the X-ray emission from these systems using archival Chandra observations of the ULX Holmberg IX X-1. This source is an ideal target to study for two reasons: it is surrounded by an optical bubble nebula with a large spatial extent ($\sim$ 400 pc) that can easily be resolved with Chandra. Further, it has a hard X-ray continuum that is easily distinguishable from the expected soft thermal emission from the nebula. However, a spectral and photometric analysis on stacked Chandra observations of the source reveals that there is no strong evidence for an X-ray bubble associated with it, to a limiting luminosity of $\sim 2 \times 10^{36}$ erg s$^{-1}$. The detection of such X-ray nebulae may be possible with future X-ray missions such as Athena, which would provide useful constraints on the kinematics of the outflow. Finally, our observations also emphasise that the nebular emission does not contribute significantly to the residuals in the X-ray spectrum of the source, which are more likely to be linked to processes localised to the ULX.