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 , extending the broadband coverage to . The observations were carried out in two epochs separated by approximately 10 days, and showed little spectral variation with an observed luminosity of erg s⁻¹. The broadband spectrum robustly confirms the presence of a clear spectral downturn above 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. [ABSTRACT FROM AUTHOR]