# Light curve and SED modeling of the gamma-ray binary 1FGL J1018.6$-$5856: constraints on the orbital geometry and relativistic flow

## TL;DR

This study models the broadband emission of the gamma-ray binary 1FGL J1018.6-5856, revealing the complex interplay of pulsar magnetosphere and intrabinary shock emissions, and constraining its orbital geometry.

## Contribution

It introduces a beamed Synchrotron-self Compton model that explains multiband light curves, improving understanding of emission mechanisms in gamma-ray binaries.

## Key findings

- Orbital modulation in X-ray and low-energy gamma-ray bands is similar.
- GeV flux likely originates from pulsar magnetosphere.
- Beamed SSC from shocked pulsar wind reproduces observed light curves.

## Abstract

We present broadband spectral energy distributions (SEDs) and light curves of the gamma-ray binary 1FGL J1018.6$-$5856 measured in the X-ray and the gamma-ray bands. We find that the orbital modulation in the low-energy gamma-ray band is similar to that in the X-ray band, suggesting a common spectral component. However, above a GeV the orbital light curve changes significantly. We suggest that the GeV band contains significant flux from a pulsar magnetosphere, while the X-ray to TeV light curves are dominated by synchrotron and Compton emission from an intrabinary shock (IBS). We find that a simple one-zone model is inadequate to explain the IBS emission, but that beamed Synchrotron-self Compton radiation from adiabatically accelerated plasma in the shocked pulsar wind can reproduce the complex multiband light curves, including the variable X-ray spike coincident with the gamma-ray maximum. The model requires inclination $\sim$50$^\circ$ and orbital eccentricity $\sim$0.35, consistent with the limited constraints from existing optical observations. This picture motivates searches for pulsations from the energetic young pulsar powering the wind shock.