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A rapidly spinning supermassive black hole at the centre of NGC 1365

Abstract

Broad X-ray emission lines from neutral and partially ionized iron observed in active galaxies have been interpreted as fluorescence produced by the reflection of hard X-rays off the inner edge of an accretion disk1,2,3,4,5,6,7. In this model, line broadening and distortion result from rapid rotation and relativistic effects near the black hole, the line shape being sensitive to its spin. Alternative models in which the distortions result from absorption by intervening structures provide an equally good description of the data8,9, and there has been no general agreement on which is correct. Recent claims10 that the black hole11,12 (2 × 106 solar masses) at the centre of the galaxy NGC 1365 is rotating at close to its maximum possible speed rest on the assumption of relativistic reflection. Here we report X-ray observations of NGC 1365 that reveal the relativistic disk features through broadened Fe-line emission and an associated Compton scattering excess of 10–30 kiloelectronvolts. Using temporal and spectral analyses, we disentangle continuum changes due to time-variable absorption from reflection, which we find arises from a region within 2.5 gravitational radii of the rapidly spinning black hole. Absorption-dominated models that do not include relativistic disk reflection can be ruled out both statistically and on physical grounds.

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Figure 1: The broadband 3–79-keV X-ray spectrum of NGC 1365.
Figure 2: X-ray spectral variability of NGC 1365.
Figure 3: Comparison between the relativistic reflection model and the multiple absorber model.
Figure 4: Error contour for the spin parameter of the supermassive black hole in NGC 1365.

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Acknowledgements

This work was supported under NASA grant number NNG08FD60C, and made use of data from the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, a project led by Caltech, 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 execution and analysis of these observations. This work also made use of observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

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Authors

Contributions

G.R. is the Principal Investigator of the XMM observations of NGC1365. He led the XMM data analysis and joint spectral analysis, modelling and interpretation of the results. F.A.H. is the Principal Investigator of NuSTAR and led observation planning, execution and participated in scientific interpretation and manuscript preparation. D.J.W. analysed NuSTAR data and participated in modelling and interpretation. K.K.M. led NuSTAR calibration analysis for NGC1365. D.S. is the NuSTAR Project Scientist and participated in definition and interpretation of the observations. B.W.G. assisted in adaptation of the NuSTAR data analysis to NGC1365. E.N. participated in XMM observations, reduction and interpretation. S.E.B., F.E.C., W.W.C., C.J.H. and W.W.Z. led efforts in design, calibration and implementation of NuSTAR. All authors participated in review of the manuscript.

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Correspondence to G. Risaliti or F. A. Harrison.

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The authors declare no competing financial interests.

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Risaliti, G., Harrison, F., Madsen, K. et al. A rapidly spinning supermassive black hole at the centre of NGC 1365. Nature 494, 449–451 (2013). https://doi.org/10.1038/nature11938

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