Transient superstrong coronal lines and broad bumps in the galaxy SDSS J074820.67+471214.3

Abstract

Variable superstrong coronal emission lines were observed in the spectrum of one galaxy, SDSSJ095209.56+214313.3, and their enigmatic origin remains controversial. In this paper, we report the detection of variable broad emission bumps reminiscent of a supernova (SN) II-Plateau spectra taken a few days after the shock breakout in a second galaxy with variable superstrong coronal lines, SDSS J074820.67+471214.3. The coronal line spectrum shows unprecedentedly high ionization with superstrong [Fe X]λ6376, [Fe XI]λ7894, [Fe XIV]λ5304, [S XII]λ7612, and [Ar XIV]λ4414, but without detectable optical [Fe VII] line emission. The coronal line luminosities are similar to those observed in bright Seyfert galaxies and 20 times more luminous than those reported in the hottest TypeIIn SN 2005ip inferred from its strong coronal lines. The coronal lines (σ ∼ 120-240kms-1) are much broader than the narrow emission lines (σ ∼40kms-1) from the star-forming regions in the galaxy, but are nearly at the same systematic redshift. We also detected a variable non-stellar continuum emission from its Sloan Digital Sky Survey spectroscopy and Galaxy Evolution Explorer photometry. 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]λ5007 intensity increased by a factor of about 10. Our analysis suggests that the coronal line region should be at least 10 light days in size and should be powered either by a steady ionizing source with a soft X-ray luminosity of at least a few 1042ergs-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. The similarity of the coronal line variability trend observed in the two galaxies suggests that the two transient events have the same origin, with SDSSJ074820.67+471214.3 being caught at an earlier stage by the spectroscopic observation. © 2011. The American Astronomical Society. All rights reserved.