SIMBAD references

Based on our long (∼300 ks) 2007 XMM-Newton observation of the Seyfert galaxy NGC 1365, we report here on the spectral and timing behavior of two ultraluminous X-ray sources (ULXs), which had previously reached isotropic X-ray luminosities L_X~ 4x10⁴⁰ erg/s (0.3-10 keV band). In 2007, they were in a lower state (L_X~ 5x10³⁹ and 1.5 x 10³⁹ erg/s for X1 and X2, respectively). Their X-ray spectra were dominated by power laws with photon indices Γ ~ 1.8 and 1.2, respectively. Thus, their spectra were similar to those at their outburst peaks. Both sources have been seen to vary by a factor of 20 in luminosity over the years, but their spectra are always dominated by a hard power law; unlike most stellar-mass black holes, they have never been found in a canonical high/soft state dominated by a standard disk. The lack of a canonical high/soft state seems to be a common feature of ULXs. We speculate that the different kind of donor star and/or a persistently super-Eddington accretion rate during their outbursts may prevent accretion flows in ULXs from settling into steady standard disks.