iPTF16fnl: a faint and fast tidal disruption event in an E+A galaxy

Abstract

We present ground-based and \textit{Swift} observations of iPTF16fnl, a likely tidal disruption event (TDE) discovered by the intermediate Palomar Transient Factory (iPTF) survey at 66.6 Mpc. The lightcurve of the object peaked at absolute $M_g=-17.2$ mag. The maximum bolometric luminosity (from optical and UV) was $L_p\simeq (1.0\pm 0.15)\times 10^{43}$ erg/s, an order of magnitude fainter than any other optical TDE discovered so far. The luminosity in the first 60 days is consistent with an exponential decay, with $L\propto e^{-(t-t_0)/\ddot{I}„}$, where $t_0$=~57631.0 (MJD) and $\ddot{I}„\simeq 15$ days. The X-ray shows a marginal detection at $L_X=2.4_{-1.1}^{1.9}\times 10^{39}$ erg/s (\textit{Swift} X-ray Telescope). No radio counterpart was detected down to 3$\ddot{I}ƒ$, providing upper limits for monochromatic radio luminosity of $\hat{I}½L_{\hat{I}}½<2.3\times 10^{36}$ erg/s and $\hat{I}½L_{\hat{I}}½<1.7\times 10^{37}$ erg/s (VLA, 6.1 and 22 GHz). The blackbody temperature, obtained from combined \textit{Swift} UV and optical photometry, shows a constant value of 19,000 K. The transient spectrum at peak is characterized by broad He II and H$\hat{I}\pm$ emission lines, with an FWHM of about 14,000 km/s and 10,000 km/s respectively. He I lines are also detected at $\hat{I}»\hat{I}»$ 5875 and 6678. The spectrum of the host is dominated by strong Balmer absorption lines, which are consistent with a post-starburst (E+A) galaxy with an age of $\sim$650 Myr and solar metallicity. The characteristics of iPTF16fnl make it an outlier on both luminosity and decay timescales, as compared to other optically selected TDEs. The discovery of such a faint optical event suggests a higher rate of tidal disruptions, as low luminosity events may have gone unnoticed in previous searches.

14 pages, 11 figures, accepted for publication in ApJ