# Soft Extragalactic X-Ray Binaries at the Eddington Threshold

## TL;DR

This study investigates four X-ray sources near the Eddington luminosity threshold, analyzing their spectral and timing properties to understand their accretion states and mechanisms, revealing diverse spectral behaviors and potential links to different ULX regimes.

## Contribution

The paper provides detailed spectral and timing analysis of four sources at the Eddington threshold, highlighting their spectral diversity and proposing physical interpretations for their accretion states.

## Key findings

- NGC 300 X-1 exemplifies the steep power-law accretion state.
- M51 ULS shows a cool blackbody spectrum, consistent with a supersoft source.
- NGC 4395 ULX-1 and NGC 6946 ULX-1 have steep power-law tails, possibly linking ULXs and ULSs.

## Abstract

The luminosity range at and just below the 10^39 erg/s cut-off for defining ultraluminous X-ray sources (ULXs) is a little-explored regime. It none-the-less hosts a large number of X-ray sources, and has great potential for improving our understanding of sources with ~Eddington accretion rates. We select a sample of four sources in this Eddington Threshold regime with good data for further study; these objects possess a variety of soft spectral shapes. We perform X-ray spectral and timing analysis on the XMM-Newton and Chandra data for these objects to gain insight into their accretion mechanisms, and also examine their optical counterparts using HST images. NGC 300 X-1 is a highly luminous and well-known example of the canonical steep power-law accretion state. M51 ULS exhibits a cool blackbody-like spectrum and is consistent with being an ultraluminous supersoft source (ULS), possibly a super-Eddington accreting object viewed at a high inclination through an optically thick outflowing wind. NGC 4395 ULX-1 and NGC 6946 ULX-1 have unusually steep power-law tails, for which we discuss a variety of possible physical mechanisms and links to similar features in Galactic microquasars, and we conclude that these sources are likely intermediate objects between the soft ultraluminous regime of ULXs and classic ULSs.