Optical follow-up of the tidal disruption event iPTF16fnl: new insights
  from X-shooter observations
  We present the results from Nordic Optical Telescope and X-shooter follow-up
campaigns of the tidal disruption event (TDE) iPTF16fnl, covering the first
$\sim$100 days after the transient discovery. We followed the source
photometrically until the TDE emission was no longer detected above the host
galaxy light. The bolometric luminosity evolution of the TDE is consistent with
an exponential decay with e-folding constant t$_{0}$=17.6$\pm$0.2 days. 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. Thanks 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 AGN 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 a 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.
