A Luminous, Fast Rising UV-Transient Discovered by ROTSE: a Tidal Disruption Event?

TL;DR

This paper reports the discovery and follow-up observations of a luminous, rapidly evolving optical transient, likely caused by a tidal disruption event of a star by a supermassive black hole, distinguished by its fast light curve and spectral features.

Contribution

It provides detailed photometric and spectroscopic data of a new UV transient, supporting its classification as a super-Eddington tidal disruption event.

Findings

Fast rise and steep decline in light curve.

Spectroscopic evidence of a host galaxy at z=0.19.

Likely tidal disruption of a star by a supermassive black hole.

Abstract

We present follow-up observations of an optical transient (OT) discovered by ROTSE on Jan. 21, 2009. Photometric monitoring was carried out with ROTSE-IIIb in the optical and Swift in the UV up to +70 days after discovery. The light curve showed a fast rise time of ~10 days followed by a steep decline over the next 60 days, which was much faster than that implied by 56Ni - 56Co radioactive decay. The SDSS DR10 database contains a faint, red object at the position of the OT, which appears slightly extended. This and other lines of evidence suggest that the OT is of extragalactic origin, and this faint object is likely the host galaxy. A sequence of optical spectra obtained with the 9.2-m Hobby-Eberly Telescope (HET) between +8 and +45 days after discovery revealed a hot, blue continuum with no visible spectral features. A few weak features that appeared after +30 days probably originated from the underlying host. Fitting synthetic templates to the observed spectrum of the host galaxy revealed a redshift of z = 0.19. At this redshift the peak magnitude of the OT is close to -22.5, similar to the brightest super-luminous supernovae; however, the lack of identifiable spectral features makes the massive stellar death hypothesis less likely. A more plausible explanation appears to be the tidal disruption of a sun-like star by the central super-massive black hole. We argue that this transient likely belongs to a class of super-Eddington tidal disruption events.