X-ray reflection in a sample of X-ray bright Ultraluminous X-ray sources

TL;DR

This study applies a reflection-based model to X-ray spectra of bright ULX sources, revealing relativistic ionized reflection features, high black hole spins, and strong light bending effects, suggesting super-Eddington accretion.

Contribution

It introduces a reflection model fitting ULX spectra without thermal disk emission, indicating near-maximal black hole spins and strong light bending effects in these sources.

Findings

Relativistically-blurred ionized reflection explains spectral drops at 6-7 keV.

Soft-excess features are consistent with disk reflection from O K and Fe L.

Black holes are inferred to have near-maximal spins and strong magnetic energy extraction.

Abstract

We apply a reflection-based model to the best available XMM-Newton spectra of X-ray bright UltraLuminous X-ray (ULX) sources (NGC 1313 X-1, NGC 1313 X-2, M 81 X-6, Holmberg IX X-1, NGC 5408 X-1 and Holmberg II X-1). A spectral drop is apparent in the data of all the sources at energies 6-7 keV. The drop is interpreted here in terms of relativistically-blurred ionized reflection from the accretion disk. A soft-excess is also detected from these sources (as usually found in the spectra of AGN), with emission from O K and Fe L, in the case of NGC 5408 X-1 and Holmberg II X-1, which can be understood as features arising from reflection of the disk. Remarkably, ionized disk reflection and the associated powerlaw continuum provide a good description of the broad-band spectrum, including the soft-excess. There is no requirement for thermal emission from the inner disk in the description of the spectra. The black holes of these systems must then be highly spinning, with a spin close to the maximum rate of a maximal spinning black hole. The results require the action of strong light bending in these sources. We suggest that they could be strongly accreting black holes in which most of the energy is extracted from the flow magnetically and released above the disc thereby avoiding the conventional Eddington limit.