Weighing the black holes in ultraluminous X-ray sources through timing

TL;DR

This paper introduces a novel timing-based method to estimate black hole masses in ultraluminous X-ray sources using the variability plane, providing evidence for intermediate-mass black holes in two ULXs.

Contribution

The paper presents a new technique leveraging the variability plane to estimate black hole masses in ULXs, specifically applying it to M82 X-1 and NGC 5408 X-1.

Findings

Black-hole mass estimates between 100 and 1300 solar masses for both ULXs.

Evidence supporting the presence of intermediate-mass black holes in these sources.

Validation of the variability plane method for ULX mass estimation.

Abstract

We describe a new method to estimate the mass of black holes in Ultraluminous X-ray Sources (ULXs). The method is based on the recently discovered ``variability plane'', populated by Galactic stellar-mass black-hole candidates (BHCs) and supermassive active galactic nuclei (AGNs), in the parameter space defined by the black-hole mass, accretion rate and characteristic frequency. We apply this method to the two ULXs from which low-frequency quasi-periodic oscillations have been discovered, M82 X-1 and NGC 5408 X-1. For both sources we obtain a black-hole mass in the range 100~1300 Msun, thus providing evidence for these two sources to host an intermediate-mass black hole.