X-Ray Spectral Cutoff and the Lack of Hard X-Ray Emission from Two Ultraluminous X-Ray Sources M81 X-6 and Holmberg IX X-1

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 and 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 >~ 3 keV with negligible contribution at high energies in the Suzaku HXD/PIN band. The 90% upper limit on the 10-30 keV band luminosity of an underlying broadband power-law component is 3.5 × 10³⁸ erg s^-1 for M81 X-6 and 1.2 × 10³⁹ erg s^-1 for Holmberg IX X-1. These limits are more than an order of magnitude lower than the bolometric (0.1-30 keV) luminosity of 6.8 × 10³⁹ erg s^-1 for M81 X-6 and 1.9 × 10⁴⁰ erg s^-1 for Holmberg IX X-1. Our results confirm earlier indications of spectral cutoffs inferred from the XMM-Newton observations of bright ULXs and show that there is not 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 nor scaled-down versions of AGNs.