The Cool Accretion Disk in ESO 243-49 HLX-1: Further Evidence of an Intermediate Mass Black Hole

TL;DR

This paper presents spectral analysis of HLX-1, providing strong evidence for an intermediate-mass black hole with mass estimates between 3,000 and 100,000 solar masses, based on relativistic accretion disk models.

Contribution

It offers the first robust spectral constraints on HLX-1's black hole mass, supporting its classification as an intermediate-mass black hole.

Findings

Mass bounds: 3,000 to 100,000 solar masses.

Spectral fits consistent with intermediate-mass black hole.

Lower bounds derived from spectral shape and Eddington limit.

Abstract

With an inferred bolometric luminosity exceeding 10^42 erg/s, HLX-1 in ESO 243-49 is the most luminous of ultraluminous X-ray sources and provides one of the strongest cases for the existence of intermediate mass black holes. We obtain good fits to disk-dominated observations of the source with BHSPEC, a fully relativistic black hole accretion disk spectral model. Due to degeneracies in the model arising from the lack of independent constraints on inclination and black hole spin, there is a factor of 100 uncertainty in the best-fit black hole mass M. Nevertheless, spectral fitting of XMM-Newton observations provides robust lower and upper limits with 3000 Msun < M < 3 x 10^5 Msun, at 90% confidence, placing HLX-1 firmly in the intermediate-mass regime. The lower bound on M is entirely determined by matching the shape and peak energy of the thermal component in the spectrum. This bound is consistent with (but independent of) arguments based solely on the Eddington limit. Joint spectral modelling of the XMM-Newton data with more luminous Swift and Chandra observations increases the lower bound to 6000 Msun, but this tighter constraint is not independent of the Eddington limit. The upper bound on M is sensitive to the maximum allowed inclination i, and is reduced to M < 10^5 Msun if we limit i < 75 deg.