Mid-InfraRed Outbursts in Nearby Galaxies (MIRONG). II. Optical Spectroscopic Follow-up

TL;DR

This study investigates mid-infrared outbursts in nearby galaxies through optical spectroscopic follow-up, revealing diverse transient phenomena including potential tidal disruption events, changing-look AGNs, and obscured sources, over a four-year period.

Contribution

It provides a large spectroscopic dataset of MIR outbursts, classifies transient sources based on spectral evolution, and highlights the diversity of SMBH-related transient phenomena.

Findings

22 out of 54 sources showed emission-line variability.

Coronal lines appeared in several sources, indicating energetic processes.

Some sources are steady, heavily obscured, or evolving into AGNs.

Abstract

Infrared echo has proven to be an effective means to discover transient accretion events of supermassive black holes (SMBHs), such as tidal disruption events (TDEs) and changing-look active galactic nuclei (AGNs), in dusty circumnuclear environments. To explore the dusty populations of SMBH transient events, we have constructed a large sample of Mid-infrared Outbursts in Nearby Galaxies (MIRONG) and performed multiwavelength observations. Here we present the results of multiepoch spectroscopic follow-up observations of a subsample of 54 objects spanning a time scale of 4 yr. Emission-line variability was detected in 22 of them with either emergence or enhancement of broad Balmer emission lines in comparison with pre-outburst spectra. Coronal lines, HeII{\lambda}4686 and Bowen line NIII{\lambda}4640 appeared in the spectra of nine,seven and two sources, respectively. These results suggest that MIRONG is a mixed bag of different transient sources. We have tentatively classified them into different subclass according to their spectral evolution and light curves. Two sources have been in a steady high broad H{\alpha} flux up to the latest observation and might be turn-on AGNs. Broad lines faded out in the remaining sources, indicating a transient ionizing source ignited by TDE or sporadic gas accretion. Thirty-one sources do not show noticeable spectral change with respect to their pre-outburst spectra. They have a statistically redder MIR color and lower MIR luminosity of the outbursts,which are consistent with heavily obscured events.