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We present X-ray spectral variability studies of the ultra-luminous X-raysource (ULX) M81 X--6 using {\it Suzaku} and {\it XMM-Newton} observationsperformed during 2001--2015. The spectra were first fitted by a standardmulti-temperature disk and a thermal Comptonization component which revealedspectral variability where the primary distinction is the change in the opticaldepth of the Comptonizing component, similar to what has been observed forother ULXs. We also fitted the spectra with a general relativistic accretiondisk emission and a power-law component and found that it can reproduce a largepart but not all of the spectral variability of the source. The parameters forthe black hole mass and spin were found to be degenerate, but the high spin andlarger mass ($20-100\,\rm M_\odot$) solutions provided near Eddington accretionrates consistent with the assumptions of the model. The spectral variation isfound to be driven by accretion rate changes leading to three differentspectral classes. Thus, our results suggest the possibility of a dominantrelativistic disk emission component for some of the spectral states of thesource.

Exploring the Spectral Variability of the Ultra-luminous X-ray source M81 X-6 with Suzaku and XMM-Newton