Abstract
We present the Pan-STARRS1 discovery of the long-lived and blue transient PS1-11af, which was also detected by GALEX with coordinated observations in the near-ultraviolet (NUV) band. PS1-11af is associated with the nucleus of an early-type galaxy at redshift z=0.4046 that exhibits no evidence for star formation or AGN activity. Four epochs of spectroscopy reveal a pair of transient broad absorption features in the UV on otherwise featureless spectra. Despite the superficial similarity of these features to P-Cygni absorptions of supernovae (SNe), we conclude that PS1-11af is not consistent with the properties of known types of SNe. Blackbody fits to the spectral energy distribution are inconsistent with the cooling, expanding ejecta of a SN, and the velocities of the absorption features are too high to represent material in homologous expansion near a SN photosphere. However, the constant blue colors and slow evolution of the luminosity are similar to previous optically-selected tidal disruption events (TDEs). The shape of the optical light curve is consistent with models for TDEs, but the minimum accreted mass necessary to power the observed luminosity is only ~0.002M_sun, which points to a partial disruption model. A full disruption model predicts higher bolometric luminosities, which would require most of the radiation to be emitted in a separate component at high energies where we lack observations. In addition, the observed temperature is lower than that predicted by pure accretion disk models for TDEs and requires reprocessing to a constant, lower temperature. Three deep non-detections in the radio with the VLA over the first two years after the event set strict limits on the production of any relativistic outflow comparable to Swift J1644+57, even if off-axis.
Highlights
When a star of mass M and radius R has an orbit with a pericenter passage sufficiently close to a black hole of mass Mbh, as in the nucleus of a galaxy, it can be torn apart by tidal forces (Hills 1975)
We have presented observations of the UV-bright transient PS1-11af, which was discovered by PS1 and detected by Galaxy Evolution Explorer (GALEX)
The transient is coincident with the nucleus of a quiescent early type galaxy with no evidence for either AGN activity or star formation
Summary
When a star of mass M and radius R has an orbit with a pericenter passage sufficiently close to a black hole of mass Mbh, as in the nucleus of a galaxy, it can be torn apart by tidal forces (Hills 1975). Most of the TDE candidates reported in the literature are large amplitude soft X-ray flares from galaxy nuclei (e.g., Bade et al 1996; Komossa & Bade 1999; Li et al 2002; Komossa et al 2004; Esquej et al 2008; Maksym et al 2013) These generally have poorly sampled light curves, but have the predicted soft spectra and light curve decay rates that are consistent with a t−5/3 decline, for suitable assumptions about the time of disruption. All quoted dates are UT, and all magnitudes are reported on the AB scale
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
