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 shape, and suggesting that some spectral features may be due to beamed disc emission around stellar-mass black holes.

Contribution

It provides new insights into the spectral variability patterns of ULXs and compares their behavior to black hole binaries, proposing a wind beaming model for the soft excess.

Findings

7 ULXs show Lx-Gamma correlation

4 ULXs fit multi-colour disc models

Soft excess follows Lsoft ∝ T^{-3.5} trend

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 disc (MCD) and a combination of these two models. We find that 7 ULXs show a correlation between the luminosity Lx and the photon index Gamma. Furthermore, 4 out of these 7 ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an Lx-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 diagrams. Finally we show that the `soft excess' reported for many of these ULXs at about 0.2 keV seems to roughly follow a trend Lsoft \propto T^{-3.5} when modelled with a power-law plus a `cool' MCD model. This is contrary to the L \propto T^4 relation that is expected from theory and what is seen for many accreting BHBs. The observed trend could instead arise from disc emission beamed by an outflowing wind around a about 10 solar mass black hole.