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

Abstract

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 ϕ = 0.2 and 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 ± 0.0004 d. The best-fitting solution to the radial velocities, held at this new period, finds the system to be more eccentric than previous observations, e = 0.531 ± 0.033 with a longitude of periastron of 151.2 ± 5.1°, and a larger mass function f = 0.00432 ± 0.00077 M⊙. 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 to 0.75 is due to the interaction of multiple randomly oriented stellar wind clumps/pulsar wind interactions around apastron.