A Hard X-Ray Study of the Ultraluminous X-Ray Source NGC 5204 X-1 with NuSTAR and XMM-Newton

We present the results from coordinated X-ray observations of the ultraluminous X-ray source NGC 5204 X-1 performed by the Nuclear Spectroscopic Telescope Array and XMM-Newton in early 2013. These observations provide the first detection of NGC 5204 X-1 above 10 {keV}, extending the broadband coverage to 0.3-20 {keV}. The observations were carried out in two epochs separated by approximately 10 days, and showed little spectral variation with an observed luminosity of {L}_{{X}}=(4.95+/- 0.11)× {10}³⁹ erg s^-1. The broadband spectrum robustly confirms the presence of a clear spectral downturn above 10 {keV} seen in some previous observations. This cutoff is inconsistent with the standard low/hard state seen in Galactic black hole binaries, as would be expected from an intermediate-mass black hole accreting at significantly sub-Eddington rates given the observed luminosity. The continuum is apparently dominated by two optically thick thermal-like components, potentially accompanied by a faint high-energy tail. The broadband spectrum is likely associated with an accretion disk that differs from a standard Shakura & Sunyaev thin disk.