  NGC 1566 is known for exhibiting recurrent outbursts, which are accompanied
by changes in spectral type. The most recent transient event occurred from 2017
to 2019 and was reported to be accompanied by a change in Seyfert
classification from Seyfert 1.8 to Seyfert 1.2. We analyze data from an optical
spectroscopic variability campaign of NGC 1566 taken with the 9.2m SALT between
July 2018 and October 2019 and supplement our data set with optical to
near-infrared spectroscopic archival data taken by VLT/MUSE in September 2015
and October 2017. We observe the emergence and fading of a strong
power-law-like blue continuum as well as strong variations in the Balmer, HeI,
HeII lines and the coronal lines [FeVII], [FeX] and [FeXI]. Moreover, we detect
broad double-peaked emission line profiles of OI 8446 and the CaII
8498,8542,8662 triplet. This is the first time that genuine double-peaked OI
8446 and CaII 8498,8542,8662 emission in AGN is reported in the literature. All
broad lines show a clear redward asymmetry with respect to their central
wavelength and we find indications for a significant blueward drift of the
total line profiles during the transient event. We show that the double-peaked
emission line profiles are well approximated by emission from a
low-inclination, relativistic eccentric accretion disk, and that single-peaked
profiles can be obtained by broadening due to scale-height dependent
turbulence. Small-scale features in the OI and CaII lines suggest the presence
of inhomogeneities in the broad-line region. We conclude that the broad-line
region in NGC 1566 is dominated by the kinematics of a relativistic eccentric
accretion disk. The broad-line region can be modeled to be vertically
stratified with respect to scale-height turbulence. The observed blueward drift
might be attributed to a low-optical-depth wind launched during the transient
event.
