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

Spider millisecond pulsars are binary systems in which a rapidly rotating neutron star interacts with a close companion, and unassociated Fermi Large Area Telescope gamma-ray sources provide a key hunting ground for new examples. This work reports the discovery of three candidate spider millisecond pulsars found through multi-wavelength follow-up of such unassociated Fermi LAT sources.

All three candidates have optical spectroscopy from the SOAR 4.1-m telescope and X-ray data from Swift. Two of the optical/X-ray sources are exceptionally strong redback candidates and lie close to the centers of the gamma-ray localization ellipses for 4FGL J1020.4-5314 and 4FGL J1408.7-3747.

Both of those optical sources are spectroscopic binaries with orbital periods of 0.15-0.3 d and mass functions of 0.6-0.9 Msun, typical of redbacks.

In 4FGL J1020.4-5314, spectra obtained over two years show bright, very broad H-alpha emission with a full width at half maximum of 2100 km/s. Remarkably, the H-alpha radial velocities are offset in phase from the absorption-line velocities of the secondary, and they match the signal expected for an emission source near the primary. This demonstrates, for the first time for any spider, that H-alpha emission can be associated with an intrabinary shock that wraps around the pulsar, opening the possibility of using optical emission lines as kinematic tracers of the intrabinary shock in some redbacks.

A third candidate spider has a less secure association with the gamma-ray source 4FGL J1429.8-0739, but it shows unusual properties: a 30.5-hour orbital period, a potential subgiant secondary, and a high X-ray luminosity. The authors conclude it is worthy of further investigation.