X-ray Variability and Hardness of ESO 243-49 HLX-1: Clear Evidence for Spectral State Transitions

TL;DR

This study presents evidence of spectral state transitions in the ULX source ESO 243-49 HLX-1, demonstrating its similarity to Galactic black hole binaries and supporting the presence of an intermediate mass black hole.

Contribution

It provides the first detailed spectral state analysis of HLX-1, confirming state transitions and disk properties indicative of an intermediate mass black hole.

Findings

Spectral state transitions similar to Galactic black hole binaries.

Luminosity scaling with disk temperature supports a thin accretion disk.

Black hole mass estimated to be >9000 solar masses.

Abstract

The ultra-luminous X-ray (ULX) source ESO 243-49 HLX-1 currently provides the strongest evidence for the existence of intermediate mass black holes. We conduct an ongoing monitoring campaign with the Swift X-ray Telescope and found that HLX-1 showed two fast rise and exponential decay with increases in the count rate of a factor ~40 separated by 375+/-13 days. We obtained new XMM-Newton and Chandra dedicated pointings that were triggered at the lowest and highest luminosities, respectively. The unabsorbed luminosities ranged from 1.9x10^40 to 1.25x10^42 erg/s. We confirm here the detection of spectral state transitions from HLX-1 reminiscent of Galactic black hole binaries: at high luminosities, the X-ray spectrum showed a thermal state dominated by a disk component with temperatures of 0.26 keV at most, and at low luminosities the spectrum is dominated by a hard power law with a photon index in the range 1.4-2.1, consistent with a hard state. The source was also observed in a steep power law state. In the thermal state, the luminosity of the disk component appears to scale with the fourth power of the inner disk temperature which supports the presence of an optically thick, geometrically thin accretion disk. The low fractional variability (rms of 9+/-9%) in this state also suggests the presence of a dominant disk. The spectral changes and long-term variability of the source cannot be explained by variations of the beaming angle and are not consistent with the source being in a super-Eddington accretion state. HLX-1 is thus an unusual ULX as it is similar to Galactic black hole binaries, which have non-beamed and sub-Eddington emission, but with luminosities 3 orders of magnitude higher. In this picture, a lower limit on the mass of the black hole of >9000 M_sun can be derived, and the disk temperature in the thermal state also suggests the presence of a black hole of a few 10^3 M_sun.