Exploring the Spectral Variability of the Ultra-luminous X-ray source M81 X-6 with Suzaku and XMM-Newton

TL;DR

This study analyzes the spectral variability of the ULX M81 X-6 over 14 years using Suzaku and XMM-Newton data, revealing changes driven by accretion rate variations and suggesting a relativistic disk component in some states.

Contribution

It provides a detailed spectral analysis of M81 X-6, exploring the role of relativistic disk emission and accretion rate changes in its variability, which is a novel insight into ULX spectral states.

Findings

Spectral variability linked to changes in Comptonization optical depth.

Relativistic disk models can explain most but not all spectral changes.

Spectral states driven by accretion rate variations, with possible relativistic disk dominance.

Abstract

We present X-ray spectral variability studies of the ultra-luminous X-ray source (ULX) M81 X--6 using {\it Suzaku} and {\it XMM-Newton} observations performed during 2001--2015. The spectra were first fitted by a standard multi-temperature disk and a thermal Comptonization component which revealed spectral variability where the primary distinction is the change in the optical depth of the Comptonizing component, similar to what has been observed for other ULXs. We also fitted the spectra with a general relativistic accretion disk emission and a power-law component and found that it can reproduce a large part but not all of the spectral variability of the source. The parameters for the black hole mass and spin were found to be degenerate, but the high spin and larger mass ($20-100\,\rm M_\odot$) solutions provided near Eddington accretion rates consistent with the assumptions of the model. The spectral variation is found to be driven by accretion rate changes leading to three different spectral classes. Thus, our results suggest the possibility of a dominant relativistic disk emission component for some of the spectral states of the source.