How well can we measure supermassive black hole spin?

Abstract

Being one of only two fundamental properties black holes possess, the spin of supermassive black holes (SMBHs) is of great interest for understanding accretion processes and galaxy evolution. However, in these early days of spin measurements, consistency and reproducibility of spin constraints have been a challenge. Here we focus on X-ray spectral modelling of active galactic nuclei (AGN), examining how well we can truly return known reflection parameters such as spin under standard conditions. We have created and fit over 4000 simulated Seyfert 1 spectra each with 375$\pm$1k counts. We assess the fits with reflection fraction of $R$ = 1 as well as reflection-dominated AGN with $R$ = 5. We also examine the consequence of permitting fits to search for retrograde spin. In general, we discover that most parameters are over-estimated when spectroscopy is restricted to the 2.5 - 10.0 keV regime and that models are insensitive to inner emissivity index and ionization. When the bandpass is extended out to 70keV, parameters are more accurately estimated. Repeating the process for $R$ = 5 reduces our ability to measure photon index ($\sim$3 to 8 per cent error and overestimated), but increases precision in all other parameters -- most notably ionization, which becomes better constrained ($\pm$45 erg cm $\rm{s^{-1}}$) for low ionization parameters ($\xi$$<$200 erg cm $\rm{s^{-1}}$). In all cases, we find the spin parameter is only well measured for the most rapidly rotating supermassive black holes (i.e. $a$ $>$ 0.8 to about $\pm$0.10) and that inner emissivity index is never well constrained. Allowing our model to search for retrograde spin did not improve the results.