Broadband X-ray Spectra of the Ultraluminous X-ray Source Holmberg IX
  X-1 observed with NuSTAR, XMM-Newton and Suzaku
  We present results from the coordinated broadband X-ray observations of the
extreme ultraluminous X-ray source Holmberg IX X-1 performed by NuSTAR,
XMM-Newton and Suzaku in late 2012. These observations provide the first
high-quality spectra of Holmberg IX X-1 above 10 keV to date, extending the
X-ray coverage of this remarkable source up to ~30 keV. Broadband observations
were undertaken at two epochs, between which Holmberg IX X-1 exhibited both
flux and strong spectral variability, increasing in luminosity from Lx =
(1.90+/-0.03)e40 erg/s to Lx = (3.35+/-0.03)e40 erg/s. Neither epoch exhibits a
spectrum consistent with emission from the standard low/hard accretion state
seen in Galactic black hole binaries, that would have been expected if Holmberg
IX X-1 harbors a truly massive black hole accreting at substantially
sub-Eddington accretion rates. The NuSTAR data confirm that the curvature
observed previously in the 3-10 keV bandpass does represent a true spectral
cutoff. During each epoch, the spectrum appears to be dominated by two
optically thick thermal components, likely associated with an accretion disk.
The spectrum also shows some evidence for a non-thermal tail at the highest
energies, which may further support this scenario. The available data allow for
either of the two thermal components to dominate the spectral evolution,
although both scenarios require highly non-standard behavior for thermal
accretion disk emission.
