Improved binary solution for the gamma-ray binary 1FGL J1018.6-5856

TL;DR

This study refines the binary system parameters of gamma-ray binary 1FGL J1018.6-5856 using new radial velocity measurements and gamma-ray data, revealing a more eccentric orbit and proposing explanations for observed emission features.

Contribution

It provides an improved binary solution with updated orbital parameters based on new optical and gamma-ray data, enhancing understanding of the system's emission mechanisms.

Findings

Orbital period determined as 16.5507 days.

Orbit is more eccentric with e=0.531.

Identified causes for X-ray and gamma-ray light curve features.

Abstract

The gamma-ray binary 1FGL J1018.6-5856 consists of an O6V((f)) type star and an unknown compact object, and shows orbitally modulated emission from radio to very high energy gamma rays. The X-ray light curve shows a maximum around the same phase as the GeV emission, but also a secondary maximum between phases $\phi=0.2 - 0.6$. A clear solution to the binary system is important for understanding the emission mechanisms occurring within the system. In order to improve on the existing binary solution, we undertook radial velocity measurements of the optical companion using the Southern African Large Telescope, as well as analysed publicly available X-ray and GeV gamma-ray data. A search for periodicity in Fermi-LAT data found an orbital period of $P=16.5507\pm0.0004$ d. The best fit solution to the radial velocities, held at this new period, finds the system to be more eccentric than previous observations, $e=0.531 \pm 0.033$ with a longitude of periastron of $151.2 \pm 5.1^\circ$, and a larger mass function $f = 0.00432\pm 0.00077$ M$_\odot$. We propose that the peaks in the X-ray and gamma-ray light curves around phase 0 are due to the observation of the confined shock formed between the pulsar and stellar wind pointing towards the observer. The secondary increase or strong rapid variations of the X-ray flux at phases 0.25-0.75 is due to the interaction of multiple randomly oriented stellar wind clumps/pulsar wind interactions around apastron.