Evidence of a Tidal-disruption Event in GSN 069 from the Abnormal Carbon and Nitrogen Abundance Ratio

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.