Long-term Spectral Variations of Ultraluminous X-ray Sources in the Interacting Galaxy Systems M 51 and NGC 4490/85

Variable ultraluminous X-ray sources (ULXs), which are considered to be black hole binaries (BHBs), are known to show state transitions similar to Galactic BHBs. However, the relation between the ULX states and the Galactic BHB states is still unclear, primarily due to the less well-understood behaviors of ULXs in contrast to the Galactic BHBs. Here, we report a statistical X-ray spectral study of 34 energy spectra from seven bright ULXs in the interacting galaxy systems M 51 and NGC 4490/85, using archive data from multiple Chandra and XMM-Newton observations spanning a few years. In order to compare them with Galactic BHB states, we applied representative spectral models of BHBs—a power-law (PL), a multi-color disk blackbody (MCD), and a slim-disk model—to all the ULX spectra. We found a hint of a bimodal structure in the luminosity distribution of the samples, suggesting that ULXs have two states that respectively have typical luminosities of (3-6)× 10³⁹ and (1.5-3)× 10³⁹ ergs s^-1. Most spectra in the brighter state are explained by the MCD or the slim-disk model, whereas those in the fainter state are explained by the PL model. In particular, the slim-disk model successfully explains the observed spectral variations of NGC 4490/85 ULX-6 and ULX-8 by changes of the mass accretion rate to a black hole of an estimated mass of <40 M _sun. From the best-fit model parameters of each state, we speculate that the brighter state in these two ULXs corresponds to the brightest state of Galactic BHBs, which is often called the "apparently standard state." The fainter state of the ULXs has a PL-shaped spectrum, but the photon index range is much wider than that seen in any single state of Galactic BHBs. We thus speculate that it is a state unique to ULXs. Some sources show much fainter and steeper spectra than the faint state, which we identified as yet another state.