TRANSIENT SUPERSTRONG CORONAL LINES AND BROAD BUMPS IN THE GALAXY SDSS J074820.67+471214.3

Abstract

Variable super-strong coronal emission lines were observed in one galaxy,\nSDSS J095209.56+214313.3, and their origin remains controversy. In this paper,\nwe report the detection of variable broad spectral bumps, reminiscent of\nsupernova (SN) II-Plateau (II-P) spectra taken a few days after the shock\nbreakout, in the second galaxy with variable super-strong coronal lines, SDSS\nJ074820.67+471214.3. The coronal line spectrum shows unprecedented high\nionization with strong [Fe X], [Fe XI], [Fe XIV], [S XII] and [Ar XIV], but\nwithout detectable optical [Fe VII] lines. The coronal line luminosities are\nsimilar to that observed in bright Seyfert galaxies, and 20 times more luminous\nthan that reported in the hottest Type IIn SN 2005ip. The coronal lines\n($\\sigma ~120-240$ km s-1) are much broader than the narrow lines ($\\sigma \\sim\n40$ km/s) from the star forming regions in the galaxy, but at nearly the same\nsystematic redshift. We also detected a variable non-stellar continuum in\noptical and UV. In the follow-up spectra taken 4-5 years later, the coronal\nlines, SN-like feature, and non-stellar continuum disappeared, while the [O\nIII] intensity increased by about a factor of ten. Our analysis suggests that\nthe coronal line region should be at least ten light days in size, and be\npowered either by a quasi-steady ionizing source with a soft X-ray luminosity\nat least a few 10^{42} erg s-1 or by a very luminous soft X-ray outburst. These\nfindings can be more naturally explained by a star tidally disrupted by the\ncentral black hole than by an SN explosion.\n