First Detection of Mid-Infrared Variability from an Ultraluminous X-Ray Source Holmberg II X-1

Ryan M. Lau (1,2), Marianne Heida (2), Mansi M. Kasliwal (2), Dominic J. Walton (3)

1 - Jet Propulsion Laboratory, California Institute of Technology,
4800 Oak Grove Drive, Pasadena, CA 91109, USA
2 - Division of Physics, Mathematics and Astronomy, Department
of Astronomy, California Institute of Technology,
Pasadena, CA 91125, USA
3 - Institute of Astronomy, Madingley Road, CB3 0HA Cambridge,
United Kingdom

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 μm 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 (Td∼600−800 K), IR luminosity (LIR∼3×10^4 L⊙), mass (Md∼1−3×10^{−6} M⊙), and equilibrium temperature radius (Req∼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] (λ=1.644 μm) 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.

Reference: Accepted by ApJ Letters
Status: Manuscript has been accepted