Discovery of superstrong, fading, iron line emission and double-peaked Balmer lines of the galaxy SDSSJ0952+2143 - the light echo of a huge flare

TL;DR

This paper reports the discovery of a galaxy exhibiting superstrong, fading iron and Balmer emission lines caused by a giant EUV--X-ray outburst, likely from a tidal disruption event, serving as a light echo to probe circumnuclear material.

Contribution

It presents the first observation of a broadband emission-line and continuum response as a light echo of a large flare in a galaxy, highlighting a new method to study circumnuclear environments.

Findings

Detection of superstrong, fading iron line emission linked to an X-ray outburst.

Observation of unusual double-peaked Balmer lines and their fading over time.

Identification of a tidal disruption event as the likely cause of the flare.

Abstract

We report the discovery of superstrong, fading, high-ionization iron line emission in the galaxy SDSSJ095209.56+214313.3 (SDSSJ0952+2143 hereafter), which must have been caused by an X-ray outburst of large amplitude. SDSSJ0952+2143 is unique in its strong multiwavelength variability; such a broadband emission-line and continuum response has not been observed before. The strong iron line emission is accompanied by unusual Balmer line emission with a broad base, narrow core and double-peaked narrow horns, and strong HeII emission. These lines, while strong in the SDSS spectrum taken in 2005, have faded away significantly in new spectra taken in December 2007. Comparison of SDSS, 2MASS, GALEX and follow-up GROND photometry reveals variability in the NUV, optical and NIR band. Taken together, these unusual observations can be explained by a giant outburst in the EUV--X-ray band, detected even in the optical and NIR. The intense and variable iron, Helium and Balmer lines represent the ``light echo'' of the flare, as it traveled through circumnuclear material. The outburst may have been caused by the tidal disruption of a star by a supermassive black hole. Spectroscopic surveys such as SDSS are well suited to detect emission-line light echoes of such rare flare events. Reverberation-mapping of these light echoes can then be used as a new and efficient probe of the physical conditions in the circumnuclear material in non-active or active galaxies.