A new way to measure supermassive black hole spin in accretion disc dominated Active Galaxies

TL;DR

This paper proposes a new method using disc continuum fitting to measure black hole spin in certain active galaxies, providing a low spin estimate for PG1244+026 and highlighting discrepancies with X-ray reflection methods.

Contribution

It introduces a novel application of disc continuum fitting to estimate black hole spin in NLS1 AGN, challenging previous high-spin results from X-ray reflection.

Findings

Disc continuum fitting indicates a low spin ($a_*<0.86$) for PG1244+026.

The method offers an alternative to X-ray reflection techniques for measuring black hole spin.

Discrepancies between methods suggest gaps in understanding of disc spectra or black hole evolution.

Abstract

We show that disc continuum fitting can be used to constrain black hole spin in a subclass of Narrow Line Seyfert 1 (NLS1) AGN as their low mass and high mass accretion rate means that the disc peaks at energies just below the soft X-ray bandpass. We apply the technique to the NLS1 PG1244+026, where the optical/UV/X-ray spectrum is consistent with being dominated by a standard disc component. This gives a best estimate for black hole spin which is low, with a firm upper limit of $a_*<0.86$. This contrasts with the recent X-ray determinations of (close to) maximal black hole spin in other NLS1 based on relativistic smearing of the iron profile. While our data on PG1244+026 does not have sufficient statistics at high energy to give a good measure of black hole spin from the iron line profile, cosmological simulations predict that black holes with similar masses have similar growth histories and so should have similar spins. This suggests that there is a problem either in our understanding of disc spectra, or/and X-ray reflection or/and the evolution of black hole spin.