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arXiv · 2503.07709

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

Rebecca Kyer·Subhroja Roy·Jay Strader·Ryan Urquhart·Isabella Molina·Peter Craig·Laura Chomiuk
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Abstract

We report the discovery of two new Galactic accreting compact objects consistent with the respective positions of the unassociated Fermi-LAT gamma-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 gamma-ray error ellipse. Both candidate counterparts show power-law spectra with photon indices $\Gamma \sim 1.7-1.9$. 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 sub-luminous 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 $\lesssim 10\%$ of the total population of transitional millisecond pulsars within 8 kpc.

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Rebecca Kyer, Subhroja Roy, Jay Strader, Ryan Urquhart, Isabella Molina, Peter Craig, Laura Chomiuk. 2025-03-10. Multiwavelength Evidence for Two New Candidate Transitional Millisecond Pulsars in the Sub-luminous Disk State: 4FGL J0639.1--8009 and 4FGL J1824.2+1231. https://doi.org/10.3847/1538-4357/adc128

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