ASCA X-ray spectra of many ULXs were described in terms of optically thick emission from hot ( kT _ { in } \sim 1 - 2 { ~ { } keV } ) accretion disks , while recent XMM-Newton and Chandra observations have revealed a cool ( kT _ { in } \sim 0.2 { ~ { } keV } ) , soft X-ray excess emission from a number of them .
Here we utilize improved calibration and high signal-to-noise XMM-Newton spectra of NGC 1313 X-1 to present evidence for a cool ( \sim 0.2 { ~ { } keV } ) soft excess and a curved or a cutoff power-law ( \Gamma \sim 1 - 1.5 , E _ { cutoff } \sim 3 - 8 { ~ { } keV } ) .
The high energy curvature may also be described by a hot ( \sim 1 - 2.5 { ~ { } keV } ) multicolor disk blackbody .
The soft excess emission is unlikely to arise from a cool disk as its blackbody temperature is similar in three XMM-Newton observations , despite a change in the observed flux by a factor of about two .
Thus , previous estimates of the black hole mass of 1000 M _ { \odot } for NGC 1313 X-1 based on the temperature of the soft excess emission is unlikely to be correct .
The power-law cutoff energy is found to decrease from \sim 8 { ~ { } keV } to \sim 3 { ~ { } keV } when the ULX brightened by a factor of about two .
The unusual spectral properties of NGC 1313 X-1 are difficult to understand in the framework of the disk/corona models generally adopted for the black hole binaries or active galactic nuclei and may require to invoke super-critical accretion rates .