X-ray absorption and occultation in LS 5039

TL;DR

This paper investigates how stellar wind absorption and occultation affect X-ray observations of LS 5039, a gamma-ray binary, and finds that an extended X-ray emission region is needed to match observations.

Contribution

It analyzes the effects of wind absorption and occultation in the pulsar wind model of LS 5039, proposing a larger X-ray emission region to explain observations.

Findings

An extended X-ray emission region (>~ 3R_star) is required.

Photoelectric absorption and occultation influence the X-ray lightcurve.

The pulsar wind scenario can explain observations with a sufficiently large emission region.

Abstract

Gamma-ray binaries are systems containing a massive star and a compact object that have been detected up to TeV energies. The high energy emission could result from particle acceleration in the region where the stellar wind from the massive star interacts with the relativistic wind from a young pulsar. LS 5039 has the most compact orbit amongst gamma-ray binaries and its X-ray lightcurve shows a stable modulation synchronized with the orbital period. Photoelectric absorption of X-rays in the O star wind and occultation of the X-ray emitting region by the massive star can alter the X-ray lightcurve and spectrum along the orbit. Yet, the X-ray spectrum and lightcurve of LS 5039 do not show intrinsic absorption or X-ray eclipses. We study these effects in the framework of the pulsar wind scenario as a function of the binary inclination angle, the stellar wind mass-loss rate and the size of the X-ray emitter. An extended X-ray emission region >~ 3R_star appears necessary to reconcile the pulsar wind scenario with observations.