Unusual 2020 Outburst of Be/X-ray Binary LXP 69.5 in the LMC

We present unexpected optical and X-ray results that challenge established models of Be/X-ray binary accretion and system geometry. Be/X-ray binaries generally consist of a neutron star in a wide, eccentric orbit around a Be star. At periastron, magnetospheric accretion from the Be disk onto the neutron star magnetic poles yields X-ray pulses with increased luminosity by orders of magnitude. These periodic X-ray outbursts often coincide with optical flares. Characterizing X-ray and optical data from these systems informs our understanding of magnetospheric accretion onto objects with extreme magnetic fields (~10¹² G) as well as the resulting feedback between the high-mass star and the orbiting pulsar. In June 2020, eROSITA detected Be/X-ray binary LXP 69.5 as the brightest transient in the Large Magellanic Cloud. We followed up with NICER to analyze the spectral and temporal properties of the neutron star over the course of the outburst. Pulse profile morphology is expected to change with accretion regime, which in turn is governed by system luminosity. However, we found the pulse profile from the peak of the outburst to be consistent with XMM-Newton observations from 2000, taken when the source was two orders of magnitude fainter. During the decay of the outburst, when the luminosity decreased by a third, the profile secondary peak became more prominent and the hardness increased. We also analyzed long-term optical data from OGLE, which spanned from March 2010 to March 2020. The orbital period of the system remains unknown, since no single period can describe the optical flares. Our best model suggests that the optical light curve can be split into three epochs with different periods (149, 171, and 200 days). Swift archival observations and our modeling of the I band quasi-periodicity suggest that the X-ray outbursts often occur near optical minimum. Such behavior is out of the ordinary for systems that host a face-on Be disk.