SIMBAD references

We present X-ray spectral variability studies of the ultraluminous X-ray source (ULX) M81 X-6 using Suzaku and XMM-Newton observations performed during 2001-2015. The spectra were first fitted by a standard multitemperature disk and a thermal Comptonization component that revealed spectral variability where the primary distinction is the change in the optical depth of the Comptonizing component, similar to what has been observed for other ULXs. We also fitted the spectra with a general relativistic accretion disk emission and a power-law component and found that it can reproduce a large part but not all of the spectral variability of the source. The parameters for the black hole mass and spin were found to be degenerate, but the high spin and larger mass (20-100 M_☉) solutions provided near-Eddington accretion rates consistent with the assumptions of the model. The spectral variation is found to be driven by accretion rate changes leading to three different spectral classes. Thus, our results suggest the possibility of a dominant relativistic disk emission component for some of the spectral states of the source.