Black Hole Spin

Despite the great progress that has been done in the last couple of decades thanks to the advancement in instrumentation and computation techniques, many open questions in AGN physics remain unsolved. For instance, the presence of a relativistic reflection in AGN and the ability of accurately measuring BH spins is still under debate. Spin measurements are a key ingredient for understanding the physical processes on scales ranging from the innermost regions of the accretion disk out to the host galaxy. Black hole spin plays a major role in understanding: the growth of SMBH, the black hole-galaxy coevolution, the production of strong outflows and relativistic jets, etc.

Testing the accuracy of reflection-based BH spin measurements in AGN

A general model of X-ray emission/absorption in AGN:

Various emission/absorption components can be present in the X-ray spectrum of any AGN. However, depending on the state in which the source is caught, we may be able to observe all or only some of these components. NGC 1365 is one of the unique sources to reveal the presence of all these components (see a nice summary by G. Risaliti, 2016).

A different approach: Blind fitting

The observer-expectancy effect may play a major role in modeling X-ray spectra. Thus, the human factor may be a hidden parameter in each fitting procedure. We decided to create a series of simulations, using instrumental responses of XMM-Newton and NuSTAR, having in mind the general model of AGN, and tried to test in which/how many cases we can recover the input parameters. This project was conducted with Emanuele Nardini (Astronomical Observatory of Arcetri) and Guido Risaliti (University of Florence).

Each simulation was created by one member of the group and fitted blindly by the two other members separately. The various spectral components mentioned above were allowed to be present or absent in any simulation (and fit), except for the primary continuum plus ionized reflection component, which was always included by construction.

We considered many factors in evaluating the success/failure of the fits, the accuracy of the results, etc.


  • Can we currently measure SMBH spins?
    Yes. In a few cases and under very special conditions.

  • Can spin-dependent X-ray signatures be identified?
    Not always. Strong reflection from the inner disc and a physical characterization
    of the corona are mandatory.

  • Will there be any improvements in the future?
    Most likely. High resolution and large area are both needed to reveal any Fe K fine
    structure and track its time variability.

  • Is there anything we can do in the mean time?
    Combine complementary diagnostics whenever possible.