Modelling the correlated keV/TeV light curves of Be/gamma-ray binaries

Be/gamma-ray binaries comprise a confirmed or presumptive pulsar orbiting a Be star and emit luminous gamma-rays. Non-thermal emissions are thought to arise from synchrotron radiation and inverse-Compton (IC) scattering in the shock where the pulsar wind is terminated by the stellar outflow. We study wind interactions and shock radiations from such systems and show that the bimodal structures observed in keV/TeV light curves are caused by enhanced synchrotron radiation and IC scattering during disc passages. We use a simple radiation model to reproduce orbital modulations of keV X-ray and TeV gamma-ray flux and compare with two confirmed pulsar/Be star binaries (i.e. PSR B1259-63/LS 2883 and PSR J2032+4127/MT91 213), and two candidates (i.e. HESS J0632+057 and LS I +61 degrees 303). We find that the keV/TeV light curves of the former two binaries can be well explained by the inclined disc model, while modelling the modulated emissions of the latter two sources remains challenging with current orbital solutions. Therefore, we propose alternative orbital geometries for HESS J0632+057 and LS I +61 degrees 303. We estimate the positions and inclination angles of Be discs by fitting correlated keV/TeV light curves. Our results could be beneficial for future measurements of orbital parameters and searches for radio pulsations from presumed pulsars.

Automated summary

Our study focuses on the wind interactions and shock radiations in Be/gamma-ray binaries, showing that the bimodal structures observed in keV/TeV light curves are caused by enhanced synchrotron radiation and IC scattering during disc passages. We use a radiation model to reproduce orbital modulations and compare with confirmed binaries, finding that the former two can be well explained while the latter two remain challenging. We propose alternative orbital geometries for the latter two sources and estimate the positions and inclination angles of Be discs. Our results could benefit future measurements and searches for radio pulsations.