Toward measuring the spin of obscured supermassive black holes

A critical assessment with disk megamasers

¹ SISSA – International School for Advanced Studies, Via Bonomea 265, 34151 Trieste, Italy
e-mail: amasini@sissa.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁴ INFN – National Institute for Nuclear Physics, Via Valerio 2, 34127 Trieste, Italy
⁵ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy

Received: 15 October 2021
Accepted: 16 November 2021

Abstract

Context. Mass and spin are two fundamental properties of astrophysical black holes. While some established indirect methods are adopted to measure both these properties of active galactic nuclei (AGN) when viewed relatively face-on, very few suggested methods exist to measure these properties when AGN are highly inclined and potentially obscured by large amounts of gas.

Aims. In this context we explore the accuracy and performance of a recently proposed method to estimate the spin of AGN by fitting the accretion disk spectral energy distribution, when adapted for highly inclined and obscured systems, and in particular to a sample of six local water megamasers. For these sources the accretion rate and inclination angle are both known, allowing us to rely only on the AGN bolometric luminosity to infer their spin.

Methods. Using the bolometric luminosity as a proxy for the accretion disk peak luminosity, we derived the expected bolometric luminosity as a function of spin. Then, we measured the bolometric luminosity of each source through X-ray spectroscopy, and compared it with the expected value to constrain the spin of the AGN.

Results. The quality of the constraints depend critically on the accuracy of the measured bolometric luminosity, which is difficult to estimate in heavily obscured systems. Three out of six sources do not show consistency between the expected and measured bolometric luminosities, while the other three (four, when considering the [OIII] line as tracer of the bolometric luminosity) are formally consistent with high spin values.

Conclusions. Our results suggest that this method, although promising (and possibly considered as a future calibrator for other methods) needs better observational data and further theoretical modeling to be successfully applied to obscured AGN and to infer robust results.