iPTF16fnl: A Faint and Fast Tidal Disruption Event in an E+A Galaxy

We present ground-based and Swift observations of iPTF16fnl, a likely tidal disruption event (TDE) discovered by the intermediate Palomar Transient Factory (iPTF) survey at 66.6 Mpc. The light curve of the object peaked at an absolute mag {M}_g=-17.2. The maximum bolometric luminosity (from optical and UV) was {L}_p≃ (1.0+/- 0.15)× {10}⁴³ erg s^-1, 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-{t₀)/τ }, where t₀ = 57631.0 (MJD) and τ ≃ 15 days. The X-ray shows a marginal detection at {L}_X={2.4}_-1.1^1.9× {10}³⁹ erg s^-1 (Swift X-ray Telescope). No radio counterpart was detected down to 3σ, providing upper limits for monochromatic radio luminosities of {ν L}_ν < 2.3× {10}³⁶ erg s^-1 and {ν L}_ν < 1.7× {10}³⁷ erg s^-1 (Very Large Array, 6.1 and 22 GHz). The blackbody temperature, obtained from combined 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 Hα emission lines, with FWHMs of about 14,000 km s^-1 and 10,000 km s^-1, respectively. He I lines are also detected at λλ 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 ∼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.