Multiwavelength evidence for a new flare-mode transitional millisecond pulsar

Abstract

We report the discovery of a new low-mass X-ray binary near the center of the unassociated Fermi GeV γ-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 Γ = 1.8 and a high ratio of X-ray to γ-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 γ-ray evidence for having undergone a state change, becoming substantially brighter in the optical and X-rays and fainter in GeV γ-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.

The MAVERIC Survey: Simultaneous Chandra and VLA observations of the transitional millisecond pulsar candidate NGC 6652B

“…NGC 6652B displays similar behaviour to other tMSPs and tMSP candidates, most prominently several X-ray flares observed over the course of the Chandra observation. This behaviour is similar to the flaring behaviour of the tMSPs PSR J1023+0038 and XSS J12270-4859, and the new tMSP candidates 3FGL J0427.9-6704 (Li et al 2020) and 4FGL J0540.0-7552 (Strader et al 2021). This prolonged flaring behaviour suggests that we are observing NGC 6652B in a flaredominated mode which is consistent with that observed from 3FGL J0427.9-6704 and 4FGL J0540.0-7552.…”

“…NGC 6652B also displays similarities to the new tMSP candidate 4FGL J0540.0-7552 (Strader et al 2021). This source displays a hard X-ray spectrum, and X-ray and optical variability.…”