Normalizing a relativistic model of Xray reflection. Definition of the reflection fraction and its implementation in relxill
Abstract
Aims: The only relativistic reflection model that implements a parameter relating the intensity incident on an accretion disk to the observed intensity is relxill. The parameter used in earlier versions of this model, referred to as the reflection strength, is unsatisfactory; it has been superseded by a parameter that provides insight into the accretion geometry, namely the reflection fraction. The reflection fraction is defined as the ratio of the coronal intensity illuminating the disk to the coronal intensity that reaches the observer.
Methods: The relxill model combines a general relativistic raytracing code and a photoionization code to compute the component of radiation reflected from an accretion that is illuminated by an external source. The reflection fraction is a particularly important parameter for relativistic models with welldefined geometry, such as the lamp post model, which is a focus of this paper.
Results: Relativistic spectra are compared for three inclinations and for four values of the key parameter of the lamp post model, namely the height above the black hole of the illuminating, onaxis point source. In all cases, the strongest reflection is produced for low source heights and high spin. A lowspin black hole is shown to be incapable of producing enhanced relativistic reflection. Results for the relxill model are compared to those obtained with other models and a Monte Carlo simulation.
Conclusions: Fitting data by using the relxill model and the recently implemented reflection fraction, the geometry of a system can be constrained. The reflection fraction is independent of system parameters such as inclination and black hole spin. The reflectionfraction parameter was implemented with the name refl_frac in all flavours of the relxill model, and the nonrelativistic reflection model xillver, in v0.4a (18 January 2016).
 Publication:

Astronomy and Astrophysics
 Pub Date:
 May 2016
 DOI:
 10.1051/00046361/201628135
 arXiv:
 arXiv:1601.03771
 Bibcode:
 2016A&A...590A..76D
 Keywords:

 accretion;
 accretion disks;
 line: profiles;
 galaxies: active;
 Xrays: galaxies;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 Published as Research Note by A&