Optical Identification of the Shortest-period Spider Pulsar System M71E

Abstract

Abstract M71E is a spider pulsar (i.e., a millisecond pulsar with a tight binary companion) with the shortest known orbital period of P = 53.3 minutes, which was discovered by Pan et al. Their favored evolutionary model suggests that it bridges between two types of spider pulsars, namely, it descended from a “redback” and will become a “black widow.” Using Hubble Space Telescope (HST) archival imaging data, we report the first optical identification of its companion COM-M71E. The HST and pulsar timing coordinates are in excellent agreement (within ∼10 mas). If M71E is associated with the globular cluster M71, our measured brightness of COM-M71E (m F606W ≈ 25.3) is broadly consistent with the expectation from Pan et al.'s preferred binary evolutionary model of a stripped dwarf companion, while it is also compatible with an ultralow-mass degenerate companion. Future multiwavelength photometric and spectroscopic observations can characterize the companion and test the evolutionary scenarios.