Multi-Epoch Multi-Wavelength Study of an Ultraluminous X-ray Source in M101: The Nature of the Secondary

Ultraluminous X-ray sources are non-nuclear point sources in external galaxies with L_X = 2 × 10³⁹-10⁴¹ erg s^-1, and thus are possibly stellar black holes with special radiation mechanisms or intermediate mass black holes of 100-10⁵ M _sun. To measure their dynamical mass, one needs to identify the secondary in the optical followed by photometric and spectroscopic monitoring. The counterpart light is usually contaminated, sometimes even dominated, by the X-ray irradiated accretion disk, which complicates the derivation of the properties of the secondary itself. Here, we report a study on M101-ULX1 with 26 Hubble Space Telescope (HST) observations and 33 X-ray observations over 16 years. There were three multi-band HST observations at different optical states over eight years, the spectral energy distributions (SEDs) of which cannot be fitted by the previously claimed secondary of a B supergiant or any other single star. All SEDs can be decomposed into the same stellar component plus a second component due to X-ray irradiation with different contributions at different optical states. The secondary in ULX1 is most likely a WR star as revealed by the SED decompositions and confirmed by the presence of He II λ4686 emission in the counterpart spectrum. Five X-ray outbursts and two optical peaks are revealed from X-ray and optical light curves, and optical peaks are identified as the optical response to X-ray outbursts. The correlated optical and X-ray variability analysis is suggestive of ellipsoidal modulation with a half-amplitude of 0.05 mag and a period in the range of 1.5-4.5 days. Future photometric and spectroscopic observations are required to confirm the orbital period and measure the dynamical mass for ULX1.