AX J0049.4 - 7323 ( SXP 756 ) 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 peculiar behaviour shown by this source , we performed the first multi-wavelength monitoring campaign during the periastron passage of December 2017 .
The monitoring lasted for about 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 ( \sim 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 .
To our knowledge , this is the second observation of a variable three-peak H \alpha profile of a Be/X-ray binary , after A0535+26 .
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 .
Further multi-wavelength observations are necessary to discriminate among the different scenarios proposed to explain the puzzling optical and X-ray properties of AX J0049.4 - 7323 .