Spectral variability of ultraluminous X-ray sources

TL;DR

This study analyzes spectral variability in 11 ultraluminous X-ray sources, revealing correlations between luminosity and spectral index, spectral pivoting, and deviations from expected theoretical relations, providing insights into their accretion states.

Contribution

It presents a detailed spectral analysis of ULXs, identifying correlations and spectral behaviors that challenge existing theoretical models and compare ULXs to black hole binaries.

Findings

7 ULXs show luminosity and photon index correlation

4 ULXs are well fitted by a multi-colour disk model

The soft excess follows an L ∝ T^{-4} trend, contrary to expectations

Abstract

We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM-Newton and Chandra observations. We use three models to describe the observed spectra; a power-law, a multi-colour disk (MCD) and a combination of these two models. We find that out of the 11 ULXs in our sample, 7 ULXs show a correlation between the luminosity and the photon index Gamma (hereafter L-Gamma correlation). Furthermore, out of the 7 ULXs that have the L-Gamma correlation, 4 ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an L-Gamma anti-correlation. The spectra of 4 ULXs in the sample can be adequately fitted with a MCD model. We compare these sources to known black hole binaries (BHB) and find that they follow similar paths in their luminosity-temperature (hereafter L-T) diagrams. Finally we show that the 'soft excess' reported for many of these ULXs at 0.2 keV seem to follow a trend L \propto T^{-4} when modeled with a power-law plus a 'cool' MCD model. This is contrary to the expected L \propto T^4 relation that is expected from theory and what is seen for many accreting BHBs.