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 kT=0.1~0.3 keV at a luminosity L>10^40 erg/s 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=~2 keV and p=~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.

5 pages, 4 figures, ApJ Letter in press; minor changes in v2 to match the published version

References in corpus (6)

• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• The Nature of SS433 and the Ultraluminous X-ray Sources
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
• On the weakness of disc models in bright ULXs
• Do Ultraluminous X-ray Sources Really Contain Intermediate-mass Black Holes?
• X-ray and optical variability of the ultraluminous X-ray source NGC 1313 X-2

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