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arXiv · 1310.0745

The ultraluminous X-ray sources NGC 1313 X-1 and X-2: a broadband study with NuSTAR and XMM-Newton

Matteo Bachetti·Vikram Rana·Dominic J. Walton·Didier Barret·Fiona A. Harrison·Steven E. Boggs·Finn E. Christensen·William W. Craig·Andrew C. Fabian·Felix Fürst·Brian W. Grefenstette·Charles J. Hailey·Ann Hornschemeier·Kristin K. Madsen·Jon M. Miller·Andrew F. Ptak·Daniel Stern·Natalie A. Webb·William W. Zhang
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Abstract

We present the results of NuSTAR and XMM-Newton observations of the two ultraluminous X-ray sources (ULX) 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 30 keV, 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.

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Matteo Bachetti, Vikram Rana, Dominic J. Walton, Didier Barret, Fiona A. Harrison, Steven E. Boggs, Finn E. Christensen, William W. Craig, Andrew C. Fabian, Felix Fürst, Brian W. Grefenstette, Charles J. Hailey, Ann Hornschemeier, Kristin K. Madsen, Jon M. Miller, Andrew F. Ptak, Daniel Stern, Natalie A. Webb, William W. Zhang. 2013-11-22. The ultraluminous X-ray sources NGC 1313 X-1 and X-2: a broadband study with NuSTAR and XMM-Newton. https://doi.org/10.1088/0004-637x%2F778%2F2%2F163

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