A Spitzer Time-Domain Investigation of Mid-Infrared Counterparts of Ultraluminous X-ray Sources

We present mid-infrared (IR) light curves of the Ultraluminous X-ray Sources (ULX) taken by the Spitzer Space Telescope at 3.6 and 4.5 _m in the Spitzer Infrared Intensive Transients Survey. We highlight the observations of the well-studied ULX Holmberg II X-1, which exhibits variability in the mid-IR. The mid-IR emission from Holmberg II X-1 is determined to arise primarily from dust emission rather than from a jet or an accretion disk outflow. A comparison of Holmberg II X-1 with a sample of spectroscopically identified massive stars in the Large Magellanic Cloud on a mid-IR color-magnitude diagram suggests that the mass donor in X-1 is a supergiant (sg) B[e]-star. The sgB[e]-interpretation is consistent with the derived dust properties and the presence of the [Fe II] (_ = 1.644 _m) emission line revealed from previous near-IR studies of X-1. We attribute the mid-IR variability of X-1 to the increased heating of dust located in a circumbinary torus. It is unclear what physical processes are responsible for the increased dust heating; however, it does not appear to be associated with the X-ray flux from the ULX, given the constant X-ray luminosities provided by serendipitous, near-contemporaneous X-ray observations around the first mid-IR variability event in 2014. Our results highlight the importance of mid-IR observations of luminous X-ray sources traditionally studied at X-ray and radio wavelengths.