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

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, HeIIλ4686 and Bowen line NIIIλ4640 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 Hα 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.

21 pages,7 figures; Submitted to ApJS; Received 2021 August 31; Revised 2021 October 13; Accepted 2021 October 25

References in corpus (27)

• The Zwicky Transient Facility: System Overview, Performance, and First Results
• Improving the full spectrum fitting method: accurate convolution with Gauss-Hermite functions
• Evolutionary Stellar Population Synthesis with MILES. Part I: The Base Models and a New Line Index System
• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• Tidal Disruption Events
• The Discovery of the First "Changing Look" Quasar: New Insights into the Physics & Phenomenology of AGN
• Seventeen Tidal Disruption Events from the First Half of ZTF Survey Observations: Entering a New Era of Population Studies
• Rapidly-Evolving and Luminous Transients from Pan-STARRS1
• Radio Properties of Tidal Disruption Events
• Optical-Ultraviolet Tidal Disruption Events
• Photometric and spectroscopic evolution of the type IIP supernova SN 2004et
• Mid-infrared variability of changing-look AGN
• iPTF Discovery of the Rapid "Turn On" of a Luminous Quasar
• A possible mechanism for multiple changing look phenomenon in Active Galactic Nuclei
• Understanding extreme quasar optical variability with CRTS: I. Major AGN flares
• A new class of flares from accreting supermassive black holes
• Infrared Echoes of Optical Tidal Disruption Events: ~1% Dust Covering Factor or Less at sub-parsec Scale
• A population of highly energetic transient events in the centres of active galaxies
• Mid-InfraRed Outburst in Nearby Galaxies (MIRONG) I: Sample Selection and Characterization
• Distinguishing Tidal Disruption Events from Impostors
• Reverberation in tidal disruption events: dust echoes, coronal emission lines, multi-wavelength cross-correlations, and QPOs
• Rapid "Turn-on" of type 1 AGN in a quiescent early type galaxy SDSS1115+0544
• AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy
• AT 2019avd: A novel addition to the diverse population of nuclear transients
• The Slow Demise of the Long-Lived SN 2005ip
• Long-term X-ray evolution of SDSS J134244.4+053056.1: A more than 18 year-old, long-lived IMBH-TDE candidate
• The Carnegie Supernova Project II. Observations of SN 2014ab possibly revealing a 2010jl-like SN IIn with pre-existing dust

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• The rate of extreme coronal line emitting galaxies in the Sloan Digital Sky Survey and their relation to tidal disruption events
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• Insights from the "Red devil" AT 2022fpx: A Dust-reddened Family of Tidal Disruption Events Excluded by Their Apparent Red Color?
• Early results in the search for extreme coronal line emitters with the Dark Energy Spectroscopic Instrument
• Radio and X-ray flux rebrightening six years after outburst in a partially-obscured extreme changing-look AGN