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We report the discovery of a variable optical and X-ray source within theerror ellipse of the previously unassociated Fermi Large Area Telescope$\gamma$-ray source 4FGL J0407.7--5702. A 22 ksec observation fromXMM-Newton/EPIC shows an X-ray light curve with rapid variability and flaring.The X-ray spectrum is well-fit by a hard power law with $\Gamma = 1.7$. Opticalphotometry taken over several epochs is dominated by aperiodic variations ofmoderate amplitude. Optical spectroscopy with SOAR and Gemini reveals a bluecontinuum with broad and double-peaked H and He emission, as expected for anaccretion disk around a compact binary. Overall, the optical, X-ray, and$\gamma$-ray properties of 4FGL J0407.7--5702 are consistent with aclassification as a transitional millisecond pulsar in the sub-luminous diskstate. We also present evidence that this source is more distant than otherconfirmed or candidate transitional millisecond pulsar binaries, and that theratio of X-ray to $\gamma$-ray flux is a promising tool to help identify suchbinaries, indicating that a more complete census for these rare systems isbecoming possible.

A New Candidate Transitional Millisecond Pulsar in the Sub-luminous Disk State: 4FGL J0407.7--5702