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

TL;DR

This study uses NuSTAR and XMM-Newton to analyze the broadband X-ray spectrum of ULX NGC 5204 X-1, revealing a spectral cutoff above 10 keV and suggesting a non-standard accretion disk structure.

Contribution

First broadband 0.3-20 keV X-ray spectrum of NGC 5204 X-1, showing a spectral cutoff inconsistent with typical black hole states, indicating a different accretion disk configuration.

Findings

Detection of NGC 5204 X-1 above 10 keV for the first time.

Spectral cutoff above 10 keV inconsistent with standard black hole states.

Spectrum dominated by two thermal-like components with a possible high energy tail.

Abstract

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)e39 erg/s. The broadband spectrum 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.