Relativistic reflection features are commonly observed in the X-ray spectra of accreting black holes. In the presence of high quality data and with the correct astrophysical model, X-ray reflection spectroscopy can potentially be quite a powerful tool to probe the strong gravity region, study the morphology of the accreting matter, measure black hole spin, and even test Einstein’s theory of general relativity in the strong field regime.

In the past 5-10 years, there has been significant progress in the development of the analysis of these features, thanks to both more sophisticated astrophysical models and new observational data. Currently, the two most advanced, publicly available, relativistic reflection models are reflkerr and relxill. Another popular choice is the non-relativistic reflection model reflionx that can describe a relativistic spectrum by employing a relativistic convolution model. All these models present some differences and have a number of simplifications preventing the possibility of performing precision measurements of black hole systems. The next generation of X-ray missions (e.g. XRISM, eXTP, Athena, STROBE-X) promises to provide unprecedented high quality data, which will necessarily require more accurate synthetic reflection spectra than those available today and a better knowledge of the physical properties of these systems. We will further need to understand the systematic uncertainties on black hole parameters that are caused by instrumental effects in current and future instruments, with a special focus on the uncertainties in our knowledge of instrument responses.

The aim of this ISSI International Team is to improve X-ray reflection spectroscopy to make it a mature technique for precision measurements of the properties of accreting black holes. The proposed team includes theorists and observers with different expertise, covering atomic physics, general relativity, accretion disk models, and X-ray data analysis techniques. In particular, the team will:

  1. identify the differences among reflkerr, relxill, and the relativistic version of reflionx, quantify their impact on the parameter estimate of black hole systems by analyzing available X-ray data of XMM-Newton and NuSTAR, and eventually try to arrive at a common model for the whole X-ray astronomy community;
  2. understand instrumental limitations (calibration and energy resolution) and their impact on the possibility of performing precision measurements of accreting black holes;
  3. investigate with simulations and real data biases introduced into the analysis of current data by shortcomings of the available reflection models;
  4. improve the current reflection models to make X-ray reflection spectroscopy capable of precision measurements of accreting black holes.

The ISSI International Team will have the opportunity to provide a substantial contribution to the development of X-ray reflection spectroscopy, which is currently the most promising method to probe the strong gravity region near the black hole event horizon using electromagnetic techniques.