The physical mechanism of radio-quiet turn-on changing-look active galactic nuclei

TL;DR

This paper proposes a new mechanism involving the collapse of an ADAF into a thin disk to explain radio-quiet changing-look AGNs, supported by timescale comparisons and a sample analysis.

Contribution

It introduces the idea that ADAF collapse can account for CL AGN variability without large-scale magnetic fields, addressing a key gap in existing models.

Findings

Cooling timescale of ADAF is shorter than observed transition timescales in most CL AGNs.

Sample analysis supports ADAF collapse as a plausible mechanism for CL AGN turn-on events.

Most observed timescales are upper limits due to limited observations.

Abstract

It is suggested that the variation of mass accretion rate in accretion disk may be responsible for the occurrence of most changing-look active galactic nuclei (CL AGNs). However, the viscous timescale of a thin disk is far longer than the observed timescale of CL AGNs. Though this problem can be resolved by introducing the large-scale magnetic field, the mechanism for radio-quiet CL AGNs with weak/absent large-scale magnetic field remains a mystery. In this work, we assume that the thin accretion disk is collapsed from the inner advection-dominated accretion flow (ADAF) instead of substituting by the outer thin disk through advection. This idea is tested by comparing the cooling timescale ($t_{\rm cool}$) of an ADAF with the observed timescale ($t_{\rm tran}$) of turn-on CL AGNs. We compile a sample of 102 turn-on CL AGNs from the archived data and calculate the cooling timescale of an ADAF with the critical mass accretion rate based on some conventional assumptions. It is found that $t_{\rm cool}$ is much shorter than $t_{\rm tran}$ in most of the CL AGNs, which validates our assumption though $t_{\rm cool}$ is not consistent with $t_{\rm tran}$ ($t_{\rm cool}<t_{\rm tran}$). However, this is reasonable since most of the CL AGNs were observed only two times, indicating that the observed timescale $t_{\rm tran}$ is the maximum value because the changing-look can indeed happen before the second observation.