NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1

Understand

We present the first broadband 0.3-25.0 kev X-ray observations of the bright ultraluminous X-ray source (ULX) Holmberg II X-1, performed by NuSTAR, XMM-Newton and Suzaku in September 2013.

• The NuSTAR data provide the first observations of Holmberg II X-1 above 10 keV, and reveal a very steep high-energy spectrum, similar to other ULXs observed by NuSTAR to date.
• These observations further demonstrate that ULXs exhibit spectral states that are not typically seen in Galactic black hole binaries.
• Comparison with other sources implies that Holmberg II X-1 accretes at a high fraction of its Eddington accretion rate, and possibly exceeds it.