Testing the Kerr Black Hole Hypothesis Using X-Ray Reflection Spectroscopy
We present the first X-ray reflection model for testing the assumption that the metric of astrophysical black holes is described by the Kerr solution. We employ the formalism of the transfer function proposed by Cunningham. The calculations of the reflection spectrum of a thin accretion disk are split into two parts: the calculation of the transfer function and the calculation of the local spectrum at any emission point in the disk. The transfer function only depends on the background metric and takes into account all the relativistic effects (gravitational redshift, Doppler boosting, and light bending). Our code computes the transfer function for a spacetime described by the Johannsen metric and can easily be extended to any stationary, axisymmetric, and asymptotically flat spacetime. Transfer functions and single line shapes in the Kerr metric are compared to those calculated from existing codes to check that we reach the necessary accuracy. We also simulate some observations with NuSTAR and LAD/eXTP and fit the data with our new model to show the potential capabilities of current and future observations to constrain possible deviations from the Kerr metric.
Additional Information© 2017. The American Astronomical Society. Received 2016 July 5; revised 2017 April 19; accepted 2017 May 19; published 2017 June 15. We thank Jiachen Jiang for useful discussions and suggestions. C.B. and S.N. were supported by the NSFC (grants U1531117 and 11305038), Fudan University (Grant No. IDH1512060), and the Thousand Young Talents Program. C.B. also acknowledges the support from the Alexander von Humboldt Foundation. A.C.-A. acknowledges funding from the Fundación Universitaria Konrad Lorenz (Project 5INV1) and thanks the Department of Physics at Fudan University for hospitality during his visit. JAG acknowledges the support of a CGPS grant from the Smithsonian Institution.
Published - Bambi_2017_ApJ_842_76.pdf
Submitted - 1607.00596