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 Mg = -17.2. The maximum bolometric luminosity (from optical and UV) was Lp ≃ (1.0 ± 0.15) × 1043erg 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 ∝ e-(t-t0)/τ, where t0 = 57631.0 (MJD) and τ ≃ 15 days. The X-ray shows a marginal detection at LX = 2.41.9 -1.1 × 1039 erg s-1 (Swift X-ray Telescope). No radio counterpart was detected down to 3σ, providing upper limits for monochromatic radio luminosities of νLν < 1.7 × 1036 erg s-1 and νLν < 2.3 × 1037 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.
Bibliographical noteFunding Information:
This work was supported by the GROWTH project funded by the National Science Foundation under grant No. 1545949. We thank the Gemini Fast Turnaround program (PI: T. Hung). S.G. is supported in part by NSF CAREER grant 1454816 and NASA Swift Cycle 12 grant NNX16AN85G. A.Y.Q.H. was supported by a National Science Foundation Graduate Research Fellowship under grant No. DGE1144469. A.J.C.T. acknowledges support from the Spanish Ministry Project AYA 2015-71718-R. Support for I.A. was provided by NASA through the Einstein Fellowship Program, grant PF6-170148. G.H. is supported by the National Science Foundation (NSF) under grant No. 1313484. This work makes use of observations from the LCO network. G.H. is supported by the National Science Foundation (NSF) under grant No. 1313484. This work makes use of observations from the LCO network. We acknowledge the use of public data from the Swift data archive. The CSS survey is funded by the National Aeronautics and Space Administration under grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the U.S. National Science Foundation under grants AST-0909182. L.A.N.L. participation in iPTF was funded by the US Department of Energy as part of the Laboratory Directed Research and Development program.
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- accretion, accretion disks
- black hole physics
- galaxies: nuclei
- stars: individual (iPTF16fnl)