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

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.

5 pages, Submitted to ApJ Letters

References in corpus (3)

• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• Quasi Periodic Oscillations and Strongly Comptonized X-ray emission from Holmberg IX X-1
• Weakly broadened iron line in the X-ray spectrum of the Ultra Luminous X-ray source M82 X-1

Cited by in corpus (8)

• The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1
• Ultraluminous X-ray sources: three exciting years
• Cutoff in the hard X-ray spectra of the ultraluminous X-ray sources HoIX X-1 and M82 X-1
• Long-term Spectral Variability of the Ultra-luminous X-ray source Holmberg IX X--1
• Impetus: Consistent SPH calculations of 3D spherical Bondi accretion onto a black hole
• Broadband X-ray Spectral Study of Ultra-luminous X-ray Source M81 X--6
• Suzaku Observations of Spectral Variations of the Ultra Luminous X-ray Source Holmberg IX X-1
• Exploring the Spectral Variability of the Ultra-luminous X-ray source M81 X-6 with Suzaku and XMM-Newton