Three New Likely Spider Millisecond Pulsar Binaries and Optical Kinematic Tracers of Intrabinary Shocks

Abstract We report the discovery of three candidate spider millisecond pulsars via multi-wavelength follow-up of unassociated Fermi Large Area Telescope γ-ray sources. All have optical spectroscopy from the SOAR 4.1-m telescope and X-ray data from Swift. Two optical/X-ray sources are exceptionally strong redback candidates, sitting close to the centres of the localization ellipses of the γ-ray sources 4FGL J1020.4–5314 and 4FGL J1408.7–3747. We show that both optical sources are spectroscopic binaries with orbital periods (0.15–0.3 d) and mass functions (0.6–0.9M⊙) typical for redbacks. 4FGL J1020.4–5314 shows bright, very broad (full width at half maximum 2100 km s−1) Hα emission in all spectra obtained over two years. Remarkably, the Hα radial velocities are offset in phase from the absorption-line velocities of the secondary, and match the signal expected for an emission source near the primary. This demonstrates, for the first time for any spider, that the Hα emission can be associated with an intrabinary shock that wraps around the pulsar, opening up the possibility of using optical emission lines as kinematic tracers of the intrabinary shock in some redbacks. A third candidate spider is less secure in its association with the γ-ray source 4FGL J1429.8–0739, but has unusual properties, including a 30.5-hour orbital period, a potential subgiant secondary, and a high X-ray luminosity, making it worthy of further investigation.