Optical flares due to X-ray irradiation during major Be X-ray binary outbursts

Context. Be X-ray binaries (BeXRBs) may show strong X-ray and optical variability, and can exhibit some of the brightest outbursts that break through the Eddington limit. Major X-ray outbursts are often accompanied by strong optical flares that evolve parallel to the X-ray outburst. Aims. Our goal is to provide a simple quantitative explanation for the optical flares with an application to a sample of the brightest outbursts of BeXRBs in the Magellanic clouds and the Galactic ULX pulsar Swift J0243.6+6124. Methods. We constructed a numerical model to study X-ray irradiation in a BeXRB system. We then conducted a parametric investigation of the model parameters and made predictions for the intensity of the optical flares based on geometric and energetic constraints. Results. From our modeling we found that the optical emission during major outbursts is consistent with being the result of X-ray irradiation of the Be disk. For individual systems, if this method is combined with independent constraints of the geometry of the Be disk, the binary orbital plane, and the plane of the observer, it can provide estimates of the Be disk size during major outbursts. Moreover, we computed a semi-analytical relation between optical flare luminosity and X-ray luminosity that is consistent with both model predictions and observed properties of flares.