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GSN 069 is an ultra-soft X-ray active galactic nucleus that previouslyexhibited a huge X-ray outburst and a subsequent long-term decay. It hasrecently presented X-ray quasi-periodic eruptions (QPEs). We report thedetection of strong nitrogen lines but weak or undetectable carbon lines in itsfar ultraviolet spectrum. With a detailed photoionization model, we use the CIV/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 partiallydisrupted red giant star can naturally explain the abnormal C/N abundance inthe UV spectrum, while the surviving core orbiting the black hole might producethe QPEs.

Evidence of a tidal disruption event in GSN 069 from the abnormal carbon and nitrogen abundance ratio