# Evidence of an X-Ray-UV Spectral Correlation in ULXs

## TL;DR

This study finds a significant anti-correlation between X-ray and UV flux ratios in ULXs, similar to AGN, suggesting a common underlying emission mechanism modeled by a multicolor accretion disk and hot corona.

## Contribution

It presents the first evidence of an X-ray-UV spectral correlation in ULXs and compares it with AGN and dwarf galaxy data, proposing a unified emission model.

## Key findings

- Significant anti-correlation between $ ext{α}_{ox}$ and UV luminosity in ULXs.
- ULX $ ext{α}_{ox}$ indices follow a steeper slope than low-z AGN.
- The correlation can be reproduced by a multicolor accretion disk with a hot corona.

## Abstract

By comparing the ratio of flux densities in the X-ray and UV wavebands by way of the spectral optical-X-Ray index, $\alpha_{ox}$, we explore the relation between the emissions in the respective wavebands for a number of ULXs with known optical counterparts. We present a significant (anti)correlation between $\alpha_{ox}$ and the L(2500 A)-UV luminosity. In comparison with low-z AGN, for which a similar correlation is observed, the ULX $\alpha_{ox}$ 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 broad-line candidate AGN 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.