The presence of a cool multicolor disk component with an inner disk temperature kT = 0.1 \sim 0.3 keV at a luminosity L > 10 ^ { 40 } ergs s ^ { -1 } has been interpreted as evidence that the ultraluminous X-ray source NGC 1313 X-2 harbors an intermediate-mass black hole ( IMBH ) .
The temperature of a disk component should vary with luminosity as L \propto T ^ { 4 } .
However , upon investigating the spectral evolution with multiple XMM-Newton observations , we found that the cool disk component failed to follow this relation with a confidence level of 0.999964 .
Indeed , the luminosity decreases as the temperature increases , and the luminosities at high temperatures are more than an order of magnitude less than expected from the L \propto T ^ { 4 } extrapolation of luminosities at low temperatures .
This places a strong constraint against the validity of modeling the X-ray spectra of NGC 1313 X-2 as emission from the accretion disk of an IMBH .
The decrease in luminosity with increasing temperature of the soft component follows the trend suggested by a model in which the soft emission arises from an outflow from a stellar-mass black hole with super-Eddington accretion viewed along the symmetry axis .
Alternatively , the spectra can be adequately fitted by a p -free disk model with kT \approx 2 keV and p \approx 0.5 .
The spectral evolution is consistent with the L \propto T ^ { 4 } relation and appears to be a high luminosity extension of the L - kT relation of Galactic black holes .
This , again , would suggest that the emission is from a super-Eddington accreting stellar mass black hole .