Testing the Performance and Accuracy of the RELXILL Model for the Relativistic X-Ray Reflection from Accretion Disks
The reflection spectroscopic model RELXILL is commonly implemented in studying relativistic X-ray reflection from accretion disks around black holes. We present a systematic study of the model's capability to constrain the dimensionless spin and ionization parameters from ~6000 Nuclear Spectroscopic Telescope Array (NuSTAR) simulations of a bright X-ray source employing the lamp-post geometry. We employ high-count spectra to show the limitations in the model without being confused with limitations in signal-to-noise. We find that both parameters are well-recovered at 90% confidence with improving constraints at higher reflection fraction, high spin, and low source height. We test spectra across a broad range—first at 10^6–10^7 and then ~10^5 total source counts across the effective 3–79 keV band of NuSTAR, and discover a strong dependence of the results on how fits are performed around the starting parameters, owing to the complexity of the model itself. A blind fit chosen over an approach that carries some estimates of the actual parameter values can lead to significantly worse recovery of model parameters. We further stress the importance to span the space of nonlinear-behaving parameters like log ξ carefully and thoroughly for the model to avoid misleading results. In light of selecting fitting procedures, we recall the necessity to pay attention to the choice of data binning and fit statistics used to test the goodness of fit by demonstrating the effect on the photon index Γ. We re-emphasize and implore the need to account for the detector resolution while binning X-ray data and using Poisson fit statistics instead while analyzing Poissonian data.
Additional Information© 2017 The American Astronomical Society. Received 2017 July 25; revised 2017 November 3; accepted 2017 November 6; published 2017 December 12. We thank the anonymous referee for the extensive suggestions in improving the paper. We also thank Sourabh Nampalliwar for helping with PyXspec issues on the clusters at the Department of Physics, Fudan University. The work of K.C. and C.B. was supported by the National Natural Science Foundation of China (NSFC, grant No. U1531117) and Fudan University (grant No. IDH1512060). K.C. also acknowledges the support from the Chinese Scholarship Council (CSC), grant No. 2015GXYD34. J.A.G. acknowledges the support from NASA, grant No. NNX15AV31G. J.F.S. was supported by Einstein Fellowship Grant PF5-160144. J.A.G. and C.B. also acknowledge the support from the Alexander von Humboldt Foundation.
Published - Choudhury_2017_ApJ_851_57.pdf
Submitted - 1711.02416.pdf