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Transient Super-strong Coronal Lines and Broad Bumps in the Galaxy SDSS J074820.67+471214.3

Variable super-strong coronal emission lines were observed in one galaxy, SDSS J095209.56+214313.3, and their origin remains controversy. In this paper, we report the detection of variable broad spectral bumps, reminiscent of supernova (SN) II-Plateau (II-P) spectra taken a few days after the shock breakout, in the second galaxy with variable super-strong coronal lines, SDSS J074820.67+471214.3. The coronal line spectrum shows unprecedented high ionization with strong [Fe X], [Fe XI], [Fe XIV], [S XII] and [Ar XIV], but without detectable optical [Fe VII] lines. The coronal line luminosities are similar to that observed in bright Seyfert galaxies, and 20 times more luminous than that reported in the hottest Type IIn SN 2005ip. The coronal lines ($σ~120-240$ km s-1) are much broader than the narrow lines ($σ\sim 40$ km/s) from the star forming regions in the galaxy, but at nearly the same systematic redshift. We also detected a variable non-stellar continuum in optical and UV. In the follow-up spectra taken 4-5 years later, the coronal lines, SN-like feature, and non-stellar continuum disappeared, while the [O III] intensity increased by about a factor of ten. Our analysis suggests that the coronal line region should be at least ten light days in size, and be powered either by a quasi-steady ionizing source with a soft X-ray luminosity at least a few 10^{42} erg s-1 or by a very luminous soft X-ray outburst. These findings can be more naturally explained by a star tidally disrupted by the central black hole than by an SN explosion.