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

TL;DR

This paper reports the discovery of a likely transitional millisecond pulsar in a sub-luminous disk state, characterized by variable optical and X-ray emissions, broad emission lines, and a high X-ray to gamma-ray flux ratio, expanding the known population.

Contribution

It identifies a new candidate transitional millisecond pulsar with multi-wavelength evidence and suggests the X-ray to gamma-ray flux ratio as a useful diagnostic for such systems.

Findings

X-ray light curve shows rapid variability and flaring.

Optical spectroscopy reveals broad, double-peaked emission lines.

The source is more distant than other known transitional millisecond pulsars.

Abstract

We report the discovery of a variable optical and X-ray source within the error ellipse of the previously unassociated Fermi Large Area Telescope $\gamma$-ray source 4FGL J0407.7--5702. A 22 ksec observation from XMM-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 $\Gamma = 1.7$. Optical photometry taken over several epochs is dominated by aperiodic variations of moderate amplitude. Optical spectroscopy with SOAR and Gemini reveals a blue continuum with broad and double-peaked H and He emission, as expected for an accretion disk around a compact binary. Overall, the optical, X-ray, and $\gamma$-ray properties of 4FGL J0407.7--5702 are consistent with a classification as a transitional millisecond pulsar in the sub-luminous disk state. We also present evidence that this source is more distant than other confirmed or candidate transitional millisecond pulsar binaries, and that the ratio of X-ray to $\gamma$-ray flux is a promising tool to help identify such binaries, indicating that a more complete census for these rare systems is becoming possible.