Multiwavelength evidence for a new flare-mode transitional millisecond pulsar

TL;DR

This paper reports the discovery of a new transitional millisecond pulsar exhibiting multiwavelength variability and a unique state change, providing insights into the behavior of such pulsars in different accretion states.

Contribution

It presents the identification of a new tMSP with evidence of a state change and persistent flare mode, expanding understanding of tMSP phenomenology.

Findings

Detected a new low-mass X-ray binary near a gamma-ray source.

Observed consistent optical, X-ray, and gamma-ray evidence of a state change in 2013.

Found the source in a persistent X-ray flare mode, similar to other tMSPs.

Abstract

We report the discovery of a new low-mass X-ray binary near the center of the unassociated Fermi GeV gamma-ray source 4FGL J0540.0-7552. The source shows the persistent presence of an optical accretion disk and exhibits extreme X-ray and optical variability. It also has an X-ray spectrum well-fit by a hard power law with a Gamma = 1.8 and a high ratio of X-ray to gamma-ray flux. Together, these properties are consistent with the classification of the binary as a transitional millisecond pulsar (tMSP) in the sub-luminous disk state. Uniquely among the candidate tMSPs, 4FGL J0540.0-7552 shows consistent optical, X-ray, and gamma-ray evidence for having undergone a state change, becoming substantially brighter in the optical and X-rays and fainter in GeV gamma-rays sometime in mid-2013. In its current sub-luminous disk state, and like one other candidate tMSP in the Galactic field, 4FGL J0540.0-7552 appears to always be in an X-ray "flare mode", indicating that this could be common phenomenology for tMSPs.