Multiwavelength Evidence for Two New Candidate Transitional Millisecond Pulsars in the Subluminous Disk State: 4FGL J0639.1-8009 and 4FGL J1824.2+1231

Abstract We report the discovery of two new Galactic accreting compact objects consistent with the respective positions of the unassociated Fermi-LAT γ -ray sources 4FGL J0639.1-8009 and 4FGL J1824.2+1231. A combination of new and archival X-ray data from Chandra, XMM-Newton, Swift/XRT, and eROSITA reveals a variable X-ray source in each γ -ray error ellipse. Both candidate counterparts show power-law spectra with photon indices Γ ∼ 1.7–1.9. Optical follow-up photometry and spectroscopy show rapid high-amplitude variability unrelated to orbital motion and persistent accretion disk spectra for both objects. We demonstrate that the properties of these X-ray/optical sources are at odds with the known phenomenology of accreting white dwarfs, but are consistent with the observed properties of the subluminous disk state of transitional millisecond pulsars. This brings the census of confirmed or candidate transitional millisecond pulsars in the Galactic field to nine. We show this potentially represents ≲10% of the total population of transitional millisecond pulsars within 8 kpc.