THE ULTRALUMINOUS X-RAY SOURCES NGC 1313 X-1 AND X-2: A BROADBAND STUDY WITH NuSTAR AND XMM–Newton

We present the results of NuSTAR and XMM–Newton observations of the two ultraluminous X-ray sources NGC 1313 X-1 and X-2. The combined spectral bandpass of the two satellites enables us to produce the first spectrum of X-1 between 0.3 and 30keV, while X-2 is not significantly detected by NuSTAR above 10 keV. The NuSTAR data demonstrate that X-1 has a clear cutoff above 10 keV, whose presence was only marginally detectable with previous X-ray observations. This cutoff rules out the interpretation of X-1 as a black hole in a standard low/hard state, and it is deeper than predicted for the downturn of a broadened iron line in a reflection-dominated regime. The cutoff differs from the prediction of a single-temperature Comptonization model. Further, a cold disk-like black body component at ∼ 0.3 keV is required by the data, confirming previous measurements by XMM–Newton only. We observe a spectral transition in X-2, from a state with high luminosity and strong variability to a lower-luminosity state with no detectable variability, and we link this behavior to a transition from a super-Eddington to a sub-Eddington regime. Subject headings: accretion, accretion disks — black hole physics — stars: black holes — X-rays: individual (NGC 1313 X-1, NGC 1313 X-2) — X-rays: stars