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Astrophysics > Astrophysics of Galaxies

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

Authors: Xin Liu (UIUC), Alexander Dittmann (UIUC), Yue Shen (UIUC), Linhua Jiang (KIAA/PKU)
Abstract: A small fraction of quasars show 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 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 yr (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, although the time baseline falls short for 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 yr (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.
Comments: 14 pages, 11 figures, ApJ resubmitted addressing referee report; comments welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1803.06362 [astro-ph.GA]
  (or arXiv:1803.06362v1 [astro-ph.GA] for this version)

Submission history

From: Xin Liu [view email]
[v1] Fri, 16 Mar 2018 18:19:26 GMT (807kb)