Optical Identification of the Shortest-Period Spider Pulsar System M71E

TL;DR

This paper reports the first optical identification of the companion to the shortest-period spider pulsar M71E using HST data, supporting evolutionary models of binary systems bridging redbacks and black widows.

Contribution

It provides the first optical detection and positional confirmation of M71E's companion, linking observational data with pulsar evolutionary theories.

Findings

Optical counterpart identified with HST (m_F606W ~ 25.3).

Coordinates match pulsar timing within ~10 mas.

Results consistent with binary evolution models.

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 min discovered by Pan et al. (2023). 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. (2023)'s preferred binary evolutionary model of a stripped dwarf companion, while it is also compatible with an ultra-low-mass degenerate companion. Future multi-wavelength photometric and spectroscopic observations can characterize the companion and test the evolutionary scenarios.