The Rate of Extreme Coronal Line Emitters and their Relation to Tidal Disruption Events

Abstract

High-ionization iron coronal lines (CLs) are a rare phenomenon observed in galaxy and quasi-stellar object spectra that are thought to be created by high-energy emission from active galactic nuclei and certain types of transients. Some galaxies, which exhibit particularly strong CLs are known as extreme coronal line emitters (ECLEs). The strength of these CLs indicates incredibly energetic progenitors interacting with some interstellar medium. In objects where the CLs are variable, the progenitor is likely an energetic transient, which could be a supernova, a variable active galactic nucleus, or a tidal disruption event (TDE). TDEs are a class of high-energy transients that occur when a star passes within the tidal radius of a supermassive black hole (SMBH). The star is ripped apart and roughly half of the matter falls onto the BH, creating a temporary accretion disc and producing a flare across the electromagnetic spectrum. The flare can then ionize the material surrounding the SMBH, creating the CLs. These variable ECLEs (vECLEs) are extremely rare, but could allow detailed characterization of the properties of the material surrounding SMBHs and the processes that create flares in TDEs.
To test whether TDEs are responsible for the CLs in vECLEs, we search through large spectro- scopic surveys for ECLEs, use follow-up spectroscopic and photometric observations to determine their variable nature, and then calculate the rate at which they occur. By comparing these rates to the- oretical and observational rates from the literature, we can constrain the proportion of TDEs that could produce CLs. We perform these searches on three spectroscopic surveys (SDSS, BOSS, and DESI), which cover overlapping redshift ranges out to z ∼ 0.4, allowing us to study the evolution of the rate of vECLEs. We discover four new vECLEs, highlighting the rarity of these transients. The rate of vECLEs is consistently one to two orders of magnitude lower than TDE rates, which suggests that only a subset of 10–40 per cent of TDEs produce strong CLs. By studying the stellar masses of the host galaxies of the vECLEs, we find evidence that vECLE rates per unit mass are higher in low mass galaxies, aligning with theoretical predictions of how TDE rates are affected by black hole (BH) masses (for which galactic stellar mass acts as a proxy). We conclude that TDEs are the most likely progenitors of vECLEs, and that detailed study of the creation and evolution of the CLs could provide valuable insight into the processes that create TDE flares. The Dark Energy Spectroscopic Instrument will provide a wealth of galaxy spectra that we estimate will contain ∼ 80 vECLEs, which will allow these links to be better established.

Date of Award	2 Jun 2025
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Or Graur (Supervisor), Becky Canning (Supervisor) & Claudia Maraston (Supervisor)