The Ultraluminous X-Ray Sources NGC 1313 X-1 and X-2: A Broadband Study with NuSTAR and XMM-Newton

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

Abstract

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 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 blackbody 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.

Additional Information

© 2013 American Astronomical Society. Received 2013 July 25; accepted 2013 October 2; published 2013 November 13. M.B. wishes to acknowledge the support from the Centre National d'Études Spatiales (CNES). This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). This work also makes use of observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, and of observations made by the Chandra X-ray Observatory. For timing analysis and plotting, a set of Python codes making use of the NumPy and Scipy libraries was used. For some plots, we used the Veusz software. The authors wish to thank Olivier Godet and Chris Done for interesting discussions, and the referee Matt Middleton, whose comments and suggestions substantively improved the quality of the manuscript.

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