We present mid-infrared ( IR ) light curves of the Ultraluminous X-ray Source ( ULX ) Holmberg II X-1 from observations taken between 2014 January 13 and 2017 January 5 with the Spitzer Space Telescope at 3.6 and 4.5 \mu m in the Spitzer Infrared Intensive Transients Survey ( SPIRITS ) .
The mid-IR light curves , which reveal the first detection of mid-IR variability from a ULX , is determined to arise primarily from dust emission rather than from a jet or an accretion disk outflow .
We derived the evolution of the dust temperature ( T _ { \mathrm { d } } \sim 600 - 800 K ) , IR luminosity ( L _ { \mathrm { IR } } \sim 3 \times 10 ^ { 4 } \mathrm { L } _ { \odot } ) , mass ( M _ { \mathrm { d } } \sim 1 - 3 \times 10 ^ { -6 } \mathrm { M } _ { \odot } ) , and equilibrium temperature radius ( R _ { \mathrm { eq } } \sim 10 - 20 AU ) .
A comparison of X-1 with a sample 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 ] ( \lambda = 1.644 \mu m ) emission line revealed from previous near-IR studies of X-1 .
We attribute the mid-IR variability of X-1 to 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 .