RELXILL_NK is a relativistic reflection model for testing Einstein’s theory of general relativity with black holes [1,2]. It is an extension of the RELXILL package [3,4] to non-Kerr spacetimes.
The study of the reflection spectrum of accretion disks around black holes is potentially a powerful tool to probe the strong gravity region of these objects. RELXILL_NK can calculate the reflection spectrum of accretion disks in parametric black hole spacetimes. Some “deformation parameters” quantify possible deviations from the Kerr background. From the comparison of X-ray data of astrophysical black holes with the theoretical predictions of RELXILL_NK we can measure these deformation parameters and check whether they vanish, as it would be required by general relativity.
The current public version of RELXILL_NK is 1.3.2 and employs the Johannsen metric with the deformation parameters alpha13 and alpha22 . The model and the associated FITS files can be downloaded from one of the following servers:
Server at Fudan University (China):
Server at the University of Tuebingen (Germany):
 Bambi et al., ApJ 842, 76 (2017)
 Abdikamalov et al., arXiv:1902.09665
 Garcia et al., ApJ, 782, 76 (2014)
 Dauser et al., MNRAS, 444, 100 (2014)
 Johannsen, PRD 88, 044002 (2013)