Multiwavelength Evidence for a New Flare-mode Transitional Millisecond Pulsar

• Published in: The Astrophysical journal 2021-08, Vol.917 (2), p.69
• Main Authors: Strader, Jay, Swihart, Samuel J., Urquhart, Ryan, Chomiuk, Laura, Aydi, Elias, Bahramian, Arash, Kawash, Adam, Sokolovsky, Kirill V., Tremou, Evangelia, Udalski, Andrej
• Format: Article
• Language: English
• Subjects: Accretion disks; Astrophysics; Gamma ray sources; Gamma rays; Millisecond pulsars; Phenomenology; Physics; Pulsars; X ray binaries; X ray spectra; X ray stars; X-ray astronomy; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac0b47

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 subluminous 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 subluminous 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.