Optical photometric monitoring of gamma-ray binaries

TL;DR

This paper discusses a systematic optical photometric monitoring program of gamma-ray binaries, aiming to understand Be star disc variations during periastron, with preliminary results for HD 215227 highlighting the importance of such data for modeling these systems.

Contribution

It introduces an ongoing systematic optical photometry monitoring program for gamma-ray binaries, providing preliminary results and emphasizing its significance for physical modeling.

Findings

Detected variability in optical photometry of gamma-ray binaries.

Preliminary results for HD 215227 show disc variability during periastron.

Highlights the need for systematic monitoring to constrain disc structures.

Abstract

Four gamma-ray binaries, namely PSR B1259-63, HESS J0632+057, HD 215227 and LS I +61 303, contain compact objects orbiting around massive Be stars. The nature of the compact object is only known in the case of PSR B1259-63, but the other systems could also contain young non-accreting pulsars with relativistic winds. Around periastron passage the compact objects should produce significant changes in the structure of the Be discs due to gravitational forces and eventually by ram pressure from the putative pulsar wind. Indeed, variability in the Halpha emission line has been detected in all these systems, and periodic variability in the optical photometry has been detected in two of them. However, there is lack of a systematic monitoring with accurate photometry, which could be used to constrain the shape of the disc during the periastron passage. This information is important to build accurate physical models to explain the broadband spectral energy distribution of these sources. Here we present an ongoing program to monitor the optical photometry of gamma-ray binaries and we show preliminary results for the case of HD 215227.