A Comparison of Two Methods for Estimating Black Hole Spin in Active Galactic Nuclei

Abstract

Angular momentum, or spin, is a fundamental property of black holes (BHs), yet it is much more difficult to estimate than mass or accretion rate (for actively accreting systems). In recent years, highquality X-ray observations have allowed for detailed measurements of the Fe Kα emission line, where relativistic line broadening allows constraints on the spin parameter (the X-ray reflection method). Another technique uses accretion disk models to fit the AGN continuum emission (the continuumfitting, or CF, method). Although each technique has model-dependent uncertainties, these are the best empirical tools currently available and should be vetted in systems where both techniques can be applied. A detailed comparison of the two methods is also useful because neither method can be applied to all AGN. The X-ray reflection technique targets mostly local (z 0.1) systems, while the CF method can be applied at higher redshift, up to and beyond the peak of AGN activity and growth. Here, we apply the CF method to two AGN with X-ray reflection measurements. For both the high-mass AGN, H1821+643, and the Seyfert 1, NGC 3783, we find a range in spin parameter consistent with the X-ray reflection measurements. However, the near-maximal spin favored by the reflection method for NGC 3783 is more probable if we add a disk wind to the model. Refinement of these techniques, together with improved X-ray measurements and tighter BH mass constraints, will permit this comparison in a larger sample of AGN and increase our confidence in these spin estimation techniques.

Observing black holes spin

“…Still, there have been several studies of SMBH spin using variants of thermal continuum fitting [49,50,51,52]. Encouragingly, for two objects that have well constrained masses as well as both X-ray reflection and thermal continuum spin measurements, the results are in good agreement [51].…”

“…Indeed, the surprising degree of optical/UV variability displayed by AGN has called into question whether a thermally-emitting standard accretion disk is even the right basic description for the optical/UV part of the spectrum [48]. Still, there have been several studies of SMBH spin using variants of thermal continuum fitting [49,50,51,52]. Encouragingly, for two objects that have well constrained masses as well as both X-ray reflection and thermal continuum spin measurements, the results are in good agreement [51].…”

A deep X-ray view of the bare AGN Ark 120

“…This model has been applied to some disc-dominated NLS1s (that is, those where R corona is close to R ISCO ) to constrain spin and mass (Done et al 2012(Done et al , 2013Jin et al 2012;Done & Jin 2016); but typically the spin value was either fixed or almost unconstrained due to large uncertainties in the BH mass. For disc-dominated objects, pure disc models can also be used for spin measurements but, until now, have only provided weak constraints due to large uncertainties on the BH mass of the considered AGN, for example SDSS J094533.99+100950.1, NGC 3783, and H1821+643 (Czerny et al 2011;Capellupo et al 2017). This latter technique is similar to the disc-continuum fitting method (which also depends on BH mass, distance and disc inclination) used for BHXBs.…”

Evidence for Higher Black Hole Spin in Radio-loud Quasars

“…Given sufficient coverage of the accretion disc SED it is also possible to constrain the spin directly from disc continuum fitting for certain objects (e.g. Done et al 2013;Capellupo et al 2016Capellupo et al , 2017.…”