We report on XMM-Newton / Chandra / Swift / HST observations of the ultraluminous X-ray source ( ULX ) in NGC 247 , which is found to make transitions between the supersoft ultraluminous ( SSUL ) regime with a spectrum dominated by a cool ( \sim 0.1 keV ) blackbody component and the soft ultraluminous ( SUL ) regime with comparable luminosities shared by the blackbody and power-law components .
Multi-epoch observations revealed an anti-correlation between the blackbody radius and temperature , R _ { bb } \propto T _ { bb } ^ { -2.8 \pm 0.3 } , ruling out a standard accretion disk as the origin of the soft X-ray emission .
The soft X-ray emission is much more variable on both short and long timescales in the SSUL regime than in the SUL regime .
We suggest that the SSUL regime may be an extension of the ultraluminous state toward the high accretion end , being an extreme case of the SUL regime , with the blackbody emission arising from the photosphere of thick outflows and the hard X-rays being emission leaked from the embedded accretion disk via the central low-density funnel or advected through the wind .
However , the scenario that the supersoft ULXs are standard ULXs viewed nearly edge-on can not be ruled out .
Flux dips on a timescale of 200 s were observed .
The dips can not be explained by an increase of absorption , but could be due to the change of accretion rate or related to thermal fluctuations in the wind or disk .
The optical emission of NGC 247 ULX exhibits a blackbody spectrum at a temperature of 19,000 K with a radius of 20 R _ { \sun } , likely arising from an OB supergiant companion star .