Evidence of an X-Ray\u2013Ultraviolet Spectral Correlation in Ultraluminous X-Ray Sources

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Abstract By comparing the ratio of flux densities in the X-ray and ultraviolet (UV) wavebands by way of the spectral optical–X-ray index, α ox, we explore the relation between the emissions in the respective wavebands for a number of ultraluminous X-ray sources (ULXs) with known optical counterparts. We present a significant (anti)correlation between α ox and the L(2500 Å)–UV luminosity. In comparison with low-z active galactic nuclei (AGNs) for which a similar correlation is observed, the ULX α ox indices follow a steeper slope albeit with a large uncertainty. The results are also compared with a small sample of dwarf-galaxy data consisting of a mixture of broadline candidate AGNs and “composites.” A number of these sources follow the steeper slope of the ULX data, potentially hinting at an intrinsic similarity of these sources to ULXs. We are able to reproduce the general trend of the ULX correlation with the use of a multicolor accretion disk coupled to a hot corona of Comptonizing electrons.