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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 [5]. 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):

Support: relxill_nk[at]

[1] Bambi et al., ApJ 842, 76 (2017)
[2] Abdikamalov et al., arXiv:1902.09665
[3] Garcia et al., ApJ, 782, 76 (2014)
[4] Dauser et al., MNRAS, 444, 100 (2014)
[5] Johannsen, PRD 88, 044002 (2013)