Bachetti, Matteo3; Rana, Vikram11; Walton, Dominic J.11; Barret, Didier3; Harrison, Fiona A.11; Boggs, Steven E.5; Christensen, Finn Erland1; Craig, William W.12; Fabian, Andrew C.7; Fuerst, Felix11; Grefenstette, Brian W.11; Hailey, Charles J.13; Hornschemeier, Ann14; Madsen, Kristin K.11; Miller, Jon M.10; Ptak, Andrew F.14; Stern, Daniel11; Webb, Natalie A.3; Zhang, William W.14
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 Université de Toulouse4 California Institute of Technology5 University of California at Berkeley6 Lawrence Livermore National Laboratory7 University of Cambridge8 Columbia University9 NASA Goddard Space Flight Center10 University of Michigan11 California Institute of Technology12 Lawrence Livermore National Laboratory13 Columbia University14 NASA Goddard Space Flight Center
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 similar to 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.
Astrophysical Journal, 2013, Vol 778, Issue 2
ASTRONOMY; MASS BLACK-HOLES; QUASI-PERIODIC OSCILLATIONS; SUPER-EDDINGTON ACCRETION; ACTIVE GALAXIES; BINARY-SYSTEMS; ESO 243-49; SLIM DISK; VARIABILITY; STATE; SPECTRA