  AX J0049.4-7323 is a Be/X-ray binary that shows an unusual and poorly
understood optical variability that consists of periodic and bright optical
outbursts, simultaneous with X-ray outbursts, characterised by a highly
asymmetric profile. The periodicity of the outbursts is thought to correspond
to the orbital period of the neutron star. To understand the behaviour shown by
this source, we performed the first multi-wavelength monitoring campaign during
the periastron passage of December 2017. The monitoring lasted for ~37 days and
consisted of X-ray, near-ultraviolet, and optical data from the Neil Gehrels
Swift Observatory, the optical I band from the OGLE survey, and spectroscopic
observations of the H-alpha line performed with the 3.9 m Anglo-Australian
Telescope. These observations revealed AX J0049.4-7323 during an anomalous
outburst having remarkably different properties compared to the previous ones.
In the I band, it showed a longer rise timescale (~60 days instead of 1-5 days)
and a longer decay timescale. At the peak of the outburst, it showed a sudden
increase in luminosity in the I band, corresponding to the onset of the X-ray
outburst. The monitoring of the H-alpha emission line showed a fast and highly
variable profile composed of three peaks with variable reciprocal brightness.
We interpreted these results as a circumstellar disc warped by tidal
interactions with the neutron star in a high eccentricity orbit during its
periastron passage. The fast jump in optical luminosity at the peak of the
outburst and the previous asymmetric outbursts might be caused by the
reprocessing of the X-ray photons in the circumstellar disc or the tidal
displacement of a large amount of material from the circumstellar disc or the
outer layers of the donor star during the periastron passage of the neutron
star, which led to an increase in size of the region emitting in the I band.
