The Ultraluminous X-Ray Sources NGC 1313 X-1 and X-2: A Broadband Study with NuSTAR and XMM-Newton
Hailey, Charles J.; Fabian, Andrew C.; Fürst, Felix; Walton, Dominic J.; Miller, Jon M.; Stern, Daniel; Madsen, Kristin K.; Hornschemeier, Ann; Boggs, Steven E.; Harrison, Fiona A.; Christensen, Finn E.; Craig, William W.; Grefenstette, Brian W.; Zhang, William W.; Bachetti, Matteo; Webb, Natalie A.; Barret, Didier; Ptak, Andrew F.; Rana, Vikram
France, United States, Denmark, United Kingdom
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.