A Candidate Tidal Disruption Event in a Quasar at z=2.359 from Abundance Ratio Variability

TL;DR

This study reports the first observation of a quasar with rapid nitrogen-to-carbon ratio variability, supporting the hypothesis that tidal disruption events cause such abundance changes, and introduces a new method for detecting TDEs at high redshift.

Contribution

It provides the first evidence of N/C variability in a quasar consistent with a TDE, and proposes a new observational approach to identify TDE candidates through spectral abundance changes.

Findings

Detected an 86% decrease in N/C ratio over 1.7 years in a quasar.

Statistical analysis shows a significant decrease in N/C in known N-rich quasars.

Optical and X-ray data support the TDE hypothesis but are not conclusive.

Abstract

A small fraction of quasars show an unusually high nitrogen-to-carbon ratio (N/C) in their spectra. These "nitrogen-rich" (N-rich) quasars are a long-standing puzzle because their interstellar medium implies stellar populations with abnormally high metallicities. It has recently been proposed that N-rich quasars may result from tidal disruption events (TDEs) of stars by supermassive black holes. The rapid enhancement of nitrogen and the depletion of carbon due to the carbon--nitrogen--oxygen cycle in supersolar mass stars could naturally produce high N/C. However, the TDE hypothesis predicts that the N/C should change with time, which has never hitherto been observed. Here we report the discovery of the first N-rich quasar with rapid N/C variability that could be caused by a TDE. Two spectra separated by 1.7 years (rest-frame) show that the N III]\lambda 1750/C III]\lambda 1909 intensity ratio decayed by ~86%+/-14% (1 \sigma). Optical (rest-frame UV) light-curve and X-ray observations are qualitatively consistent with the TDE hypothesis; though, the time baseline falls short of a definitive proof. Putting the single-object discovery into context, statistical analyses of the ~80 known N-rich quasars with high-quality archival spectra show evidence (at a 5\sigma\ significance level) of a decrease in N/C on timescales of >1 year (rest-frame) and a constant level of ionization (indicated by the C III]\lambda 1909/C IV\lambda 1549 intensity ratio). If confirmed, our results demonstrate the method of identifying TDE candidates in quasars via abundance ratio variability, opening a new window of TDE observations at high redshift (z>2) with upcoming large-scale time-domain spectroscopic surveys.