The high energy properties of the Galactic X-ray binary population, as seen with the Fermi Large Area Telescope

Abstract

The Fermi Large Area Telescope (Fermi-LAT) is a space instrument which has been in orbit since June 2008, and is sensitive to photons from 30 MeV to approximately 1 TeV. I use the Fermi-LAT data, alongside X-ray and optical data to perform an investigation of the high energy γ-ray properties of the X-ray binaries, stellar systems where either a neutron star or a black hole is in a close orbit with a main sequence star. The complex case of the low mass X-ray binary, V404 Cygni, is looked at; it is determined that there is no evidence for γ-ray emission from this system. The majority of the remaining Galactic X-ray binary population is then surveyed, and evidence for γ-ray emission is found from 5 systems: the high mass X-ray binaries SAX J1324.4-6200, GRO J1008-57, 1A 0535+262 and RX J2030.5+4751 and the low mass X-ray binary GS 1826-238. There are a number of additional systems where spatially coincident γ-ray excesses are identified, however evidence to associate these with their respective X-ray binaries is weak. I examine these systems alongside the X-ray binaries with known γ-ray emission and determine that γ-ray emission in high mass X-ray binaries is highly dependent on orbital phase, and X-ray accretion state so that many undetected systems may be weakly emitting transients. In the case of the low mass systems, I determine that these are nearby sources, and that almost all are known transitional millisecond pulsars. Future observatories such as CTA, SWGO and AMEGO are likely to discover more γ-ray emitting X-ray binaries, and these facilities are discussed.