2019MNRAS.488.4614S

Optical and radio observations of shock-ionized bubble nebulae surrounding ultraluminous X-ray sources (ULXs) suggest that they are powered by jets or supercritical 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 <=10³⁶ 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 (Ho 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 (∼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 ∼2 x 10³⁶ erg s^–1. The detection of such X-ray nebulae may be possible with future X-ray missions such as Advanced Telescope for High ENergy Astrophysics(ATHENA), which would provide useful constraints on the kinematics of the outflow. Finally, our observations also emphasize 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 localized to the ULX.