The Changing-look Phenomenon Accompanied by an Accretion Mode Transition in NGC 3786

TL;DR

This study investigates the changing-look phenomenon in NGC 3786, linking it to accretion mode transitions and torus covering factor variations, based on long-term optical and near-infrared spectral monitoring.

Contribution

It provides evidence that the CL phenomenon is primarily driven by changes in line-of-sight extinction related to accretion mode, rather than tidal disruption events.

Findings

Mid-infrared brightness increased by 1-1.5 mag since CL onset.

Broad Paβ and Paα lines strengthened over two years.

Broad Hα flux remained unchanged.

Abstract

To reveal the physical origin of the changing-look (CL) phenomenon in NGC 3786, which transitioned from type 1.8/1.9 to type 1, we present an analysis of long-term spectral monitoring in the optical and near-infrared obtained with Gemini/GMOS-N and Gemini/GNIRS, respectively. Since the onset of the CL phenomenon, NGC 3786 has remained $\sim 1-1.5$ mag brighter in the mid-infrared than in the pre-CL stage, whereas the optical continuum has changed only moderately ($\sim 0.2-0.3$ mag). Spectroscopic analysis further reveals that while the fluxes of the broad Pa$\beta$ and Pa$\alpha$ lines were enhanced over a two-year follow-up period, the flux of the broad H$\alpha$ line remained unchanged. We propose that observed temporal variations in the continuum and line flux ratios disfavor a tidal disruption event origin. Instead, the observations can be primarily explained by a gradual change in line-of-sight extinction driven by variations in the torus covering factor, which is determined by the Eddington ratio and the accretion mode. An additional mechanism, arising from the physical conditions within the broad-line region, may partially account for the temporal evolution of the flux ratios. Our study highlights the importance of investigating the CL phenomenon in intermediate-type active galactic nuclei associated with outbursts detected only in the mid-infrared to explore the detailed structural evolution of nuclear activity.