Spectral Evolution of NGC 1313 X-2: Evidence against the Cool Disk Model

Abstract

The presence of a cool multicolor disk component with an inner disk temperature keV at a kT p∼ 0.1-0.3 luminosity ergs s 1 has been interpreted as evidence that the ultraluminous X-ray source NGC 1313 X-2 40 L 1 10 harbors an intermediate-mass black hole (IMBH). The temperature of a disk component should vary with luminosity as. However, upon investigating the spectral evolution with multiple XMM-Newton observations, we found 4 L ∝ T 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 extrapolation of luminosities at low temperatures. This places a strong constraint 4 L ∝ T 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 keV and. The spectral evolution is consistent with the relation and appears to be a 4 kT ≈ 2 p ≈ 0.5 L ∝ T 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.