AGN accretion disks as spatially resolved by polarimetry

TL;DR

This paper demonstrates that optical and near-infrared polarization measurements can isolate the intrinsic emission spectrum of AGN accretion disks, revealing spectral features that confirm the theoretical model of optically thick, locally heated disks.

Contribution

It introduces a novel polarization-based method to observe AGN accretion disk spectra, overcoming the obscuration by surrounding regions and providing empirical verification of disk models.

Findings

Detection of Balmer edge in absorption in polarized flux

Blue near-infrared spectral shape consistent with theoretical predictions

Verification of the optically thick, locally heated disk model in AGNs

Abstract

A crucial difficulty in understanding the nature of the putative accretion disk in AGNs is that some of its key intrinsic spectral signatures cannot be observed directly. The strong emissions from the broad-line region (BLR) and the obscuring torus, which are generally yet to be spatially resolved, essentially 'bury' such signatures. Here we argue that we can actually isolate the disk emission spectrum by using optical and near-infrared polarization of quasars and uncover the important spectral signatures. In these quasars, the polarization is considered to originate from electron scattering interior to the BLR, so that the polarized flux shows the disk spectrum with all the emissions from the BLR and torus eliminated. The polarized flux observations have now revealed a Balmer edge feature in absorption and a blue near-infrared spectral shape consistent with a specific and robust theoretical prediction. These results critically verify the long-standing picture of an optically thick and locally heated disk in AGNs.