X-ray spectral cut-off and the lack of hard X-ray emission from two ultraluminous X-ray sources M81 X-6 and Holmberg IX X-1

TL;DR

This study analyzes the broadband X-ray spectra of two ultraluminous X-ray sources, revealing spectral cut-offs above 3 keV and a lack of high-energy emission, challenging their classification as scaled-up black hole binaries or scaled-down AGNs.

Contribution

First broadband spectral analysis of M81 X-6 and Holmberg IX X-1 showing spectral cut-offs and absence of high-energy power-law components, providing new insights into ULX emission mechanisms.

Findings

Spectral cut-offs occur above 3 keV in both ULXs.

No significant high-energy power-law component detected.

ULX spectra differ from those of Galactic black hole binaries and AGNs.

Abstract

We present broadband X-ray spectral study of two ultraluminous X-ray sources (ULXs), M81 X-6 and Holmberg IX X-1 based on Suzaku & XMM-Newton observations. We perform joint broadband spectral analysis of the brightest sources in the field, i.e. the two ULXs and the active galactic nucleus (AGN) in M81, and demonstrate that the X-ray spectra of the ULXs cut off at energies more than about 3keV with negligible contribution at high energies in the Suzaku HXD/PIN band. The 90% upper limit on the 10-30keV band luminosity of an underlying broadband power-law component is 3.5e38 ergs/s for M81 X-6 and 1.2e39ergs/s for Holmberg IX X-1. These limits are more than an order of magnitude lower than the bolometric (0.1-30 keV) luminosity of 6.8e39ergs/s for M81 X-6 and 1.9e40 ergs/s for Holmberg IX X-1. Our results confirm earlier indications of spectral cut-offs inferred from the XMM-Newton observations of bright ULXs and show that there isn't an additional high energy power-law component contributing significantly to the X-ray emission. The spectral form of the two ULXs are very different from those of Galactic black hole X-ray binaries (BHBs) or AGNs. This implies that the ULXs are neither simply scaled-up versions of stellar mass BHBs or scaled-down versions of AGNs.