AGN Light Echoes and the Accretion Disc Self-Gravity Limit

TL;DR

This paper links observed AGN light echo delays to the theoretical self-gravity limit of accretion discs, providing a new way to test disc models and estimate redshifts of distant AGN.

Contribution

It presents observational evidence that AGN light echo delays align with the predicted self-gravity radius, offering a novel test of accretion disc theory.

Findings

Observed AGN light echoes are near 12 days in the rest frame.

Longer observed lags imply higher redshifts for distant AGN.

Supports the self-gravity limit as a key feature of AGN discs.

Abstract

Accretion disc theory predicts that an AGN disc becomes self-gravitating and breaks up into stars at an outer radius $R_{\rm sg}$ ~ 12 light-days, with effectively no free parameter. We present evidence that the longer observed AGN light echoes are all close to 12d in the AGN rest frames. These observations give a stringent test of AGN disc theory. Further monitoring should offer insight into the formation angular momentum of the gas forming the disc. For distant AGN, observed lags significantly longer than 12d give lower limits on their redshifts.