Measuring the black hole mass in Ultraluminous X-ray Sources with the X-ray Scaling Method

TL;DR

This paper applies a novel X-ray scaling method to ultraluminous X-ray sources, estimating their black hole masses and confirming the method's scale-independence across different black hole systems.

Contribution

The study extends the X-ray scaling method from galactic and supermassive black holes to ULXs, providing new mass estimates and validating the method's applicability.

Findings

ULXs host both stellar and intermediate mass black holes.

Most ULXs are consistent with highly accreting massive stellar black holes (~100 solar masses).

The X-ray scaling method agrees with other mass estimation techniques.

Abstract

In our recent work, we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes, could be reliably extended to estimate the mass of supermassive black holes accreting at moderate to high level. Here, we apply this X-ray scaling method to ultraluminous X-ray sources (ULXs) to constrain their \mbh. Using 49 ULXs with multiple \xmm\ observations, we infer that ULXs host both stellar mass BHs and intermediate mass BHs. The majority of the sources of our sample seem to be consistent with the hypothesis of highly accreting massive stellar BHs with $M_{\rm BH}\sim100\,M_{\odot}$. Our results are in general agreement with the \mbh\ values obtained with alternative methods, including model-independent variability methods. This suggests that the X-ray scaling method is an actual scale-independent method that can be applied to all BH systems accreting at moderate-high rate.