A Hard X-ray Study of 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 NuSTAR 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 LX = (4.95 ± 0.11) × 10 39 erg s. 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. Subject headings: Black hole physics – X-rays: binaries – X-rays: individual (NGC 5204 X-1)