Optical follow-up of the tidal disruption event iPTF16fnl: new insights from X-shooter observations

Abstract

We present the results from Nordic Optical Telescope and X-shooter follow-up campaigns of the tidal disruption event (TDE) iPTF16fnl, covering the first similar to 100 d after the transient discovery. We followed the source photometrically until the TDE emission was no longer detected above the host galaxy light. The bolometric luminosity evolution of the TDE is consistent with an exponential decay with e-folding constant t(0) = 17.6 +/- 0.2 d. The early-time spectra of the transient are dominated by broad He II lambda 4686, H beta, H alpha, and N III lambda 4100 emission lines. The latter is known to be produced together with the N III lambda 4640 in the Bowen fluorescence mechanism. Due to the medium-resolution X-shooter spectra we have been able to separate the Bowen blend contribution from the broad He II emission line. The detection of the Bowen fluorescence lines in iPTF16fnl place this transient among the N-rich TDE subset. In the late-time X-shooter spectra, narrow emission lines of [O III] and [N II] originating from the host galaxy are detected, suggesting that the host galaxy harbours a weak active galactic nucleus in its core. The properties of all broad emission lines evolve with time. The equivalent widths follow an exponential decay compatible with the bolometric luminosity evolution. The full width at half-maximum of the broad lines decline with time and the line profiles develop a narrow core at later epochs. Overall, the optical emission of iPTF16fnl can be explained by being produced in an optically thick region in which high densities favour the Bowen fluorescence mechanism and where multiple electron scatterings are responsible for the line broadening.