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

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 ($t_{\mathrm{cool}}$) of an ADAF with the observed timescale ($t_{\mathrm{tran}}$) 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 $t_{\mathrm{cool}}$ is much shorter than $t_{\mathrm{tran}}$ in most of the CL AGNs, which validates our assumption though $t_{\mathrm{cool}}$ is not consistent with $t_{\mathrm{tran}}$ ($t_{\mathrm{cool}}<t_{\mathrm{tran}}$). However, this is reasonable since most of the CL AGNs were observed only two times, indicating that the observed timescale $t_{\mathrm{tran}}$ is the maximum value because the changing-look can indeed happen before the second observation.

7 pages, 2 figures, accepted for publication in ApJ