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

TL;DR

This paper models the keV/TeV light curves of Be/gamma-ray binaries, explaining observed bimodal structures through wind interactions and shock radiation, and proposes alternative orbital geometries for certain sources.

Contribution

It introduces a simple radiation model that reproduces orbital modulations and suggests new orbital configurations for specific Be/gamma-ray binaries.

Findings

KeV/TeV light curves explained by inclined disc model for confirmed binaries.

Modeling challenges for candidate binaries highlight need for revised orbital parameters.

Estimated Be disc positions and inclinations aid future orbital and pulsation studies.

Abstract

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$^{\circ}$303). We find that the keV/TeV light curves of the former two binaries can be well explained by the inclined disc model, while modelling the modulated emissions of the latter two sources remains challenging with current orbital solutions. Therefore, we propose alternative orbital geometries for HESS J0632+057 and LS I +61$^{\circ}$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.