Evidence of a Tidal-disruption Event in GSN 069 from the Abnormal Carbon and Nitrogen Abundance Ratio
Zhenfeng Sheng, Tinggui Wang, G. J. Ferland, Xinwen Shu, Chenwei Yang, Ning Jiang, Yang Chen
Abstract
Abstract GSN 069 is an ultra-soft X-ray active galactic nucleus that previously exhibited a huge X-ray outburst and a subsequent long-term decay. It has recently presented X-ray quasi-periodic eruptions (QPEs). We report the detection of strong nitrogen lines but weak or undetectable carbon lines in its far-ultraviolet spectrum. With a detailed photoionization model, we use the C iv /N iv ] ratio and other ratios between nitrogen lines to constrain the [C/N] abundance of GSN 069 to be from −3.33 to −1.91. We argue that a partially disrupted red giant star can naturally explain the abnormal C/N abundance in the UV spectrum, while the surviving core orbiting the black hole might produce the QPEs.