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 outburst and subsequent long-term decay. It has recently presented quasi-periodic eruptions (QPEs). We report the detection of strong nitrogen lines but weak or undetectable carbon in its far ultraviolet spectrum. With detailed photoionization model, we use C IV/N IV] ratio other ratios between to constrain [C/N] abundance be from -3.33 -1.91. argue partially disrupted red giant star can naturally explain abnormal C/N UV spectrum, while surviving core orbiting black hole might produce QPEs.