NuSTAR, XMM-Newton, and Suzaku Observations of the Ultraluminous X-Ray Source Holmberg II X-1
2015
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 2013 September. 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. The soft Xray
spectrum (E < 10 keV) appears to be dominated by two blackbody-like emission components, the hotter of
which may be associated with an accretion disk. However, all simple disk models under-predict the NuSTAR data
above ∼10 keV and require an additional emission component at the highest energies probed, implying the
NuSTAR data does not fall away with a Wien spectrum. We investigate physical origins for such an additional
high-energy emission component and favor a scenario in which the excess arises from Compton scattering in a hot
corona of electrons with some properties similar to the very high state seen in Galactic binaries. The observed
broadband 0.3–25.0 keV luminosity inferred from these epochs is L_X (8.1 0.1) 1039 =±× erg s^(−1), typical for
Holmberg II X^(-1), with the majority of this flux (∼90%) emitted below 10 keV.
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