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

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 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 ∝ 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 ∝ 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 ∝ 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. 1. INTRODUCTION Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of a 20 M ⊙ black hole (3 × 10 39 ergs s −1 ; for a review, see Fabbiano & White 2006). Strong variability from many ULXs indicates they are compact objects. If the X-rays are emitted isotropically below the Eddington limit, then the high luminosity would indicate that ULXs contain IMBHs (Colbert & Mushotzky 1999; Kaaret et al. 2001). However, the emission could be mechanically or relativistically beamed (King et al. 2001; Körding et al. 2002), or exceed the Eddington limit (Begelman 2002; Ebisawa et al. 2003); in these cases, IMBHs are not required. The sum of a power-law and a multicolor disk (MCD; Shakura & Sunyaev 1973) has been found to describe well the X-ray spectra of black hole binaries. …