Polarimetry of Ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray emitters exceeding the Eddington luminosity, offering key insights into extreme accretion physics. Their origin is attributed to either sub-Eddington accretion onto intermediate-mass black holes (IMBHs) or supercritical accretion onto stellar-mass compact objects in binary systems. A subset of ULXs shows radio jets and surrounding bubbles, indicative of strong mechanical feedback, but no polarization detection has been reported to date because of limited sensitivity and angular resolution at multi-megaparsec distances. Next-generation facilities such as the Square Kilometre Array (SKA) will enable polarimetric studies of ULX outflows and bubbles, providing vital information on their magnetic field structure and jet-ISM interactions. Using Holmberg II X-1 (3.39 Mpc) as a representative system, we evaluated the detectability of its polarized jet and bubble with the SKA-Mid AA4 configuration. Assuming a fractional polarization of 20% and Band 2 observations centered at 1310 MHz, the expected polarized intensities are 260 $\mu$Jy beam$^{-1}$ for the jet and 20 $\mu$Jy beam$^{-1}$ for the bubble. A Holmberg II X-1-like jet is detectable within 10 hours even at $\sim$ 10 Mpc, while bubble polarization requires $\sim$ 100 hours. Although resolving fine structures is feasible only for nearby ULXs, future SKA-VLBI capabilities will help overcome this limitation and enable detailed mapping of magnetic field morphology. These results indicate that the SKA will enable the first detection of polarized emission from ULXs, advancing our understanding of magnetic field geometry, jet formation, and feedback in super- and sub-Eddington accretors.