The Nature of the Mysterious Companion to PSR J1928+1815

PSR J1928+1815 is a 10.55 ms millisecond pulsar in a 3.6 hr orbit with a 1.0-1.6 solar mass companion that produces extended radio eclipses. The companion, proposed to be a stripped helium star, is undetected in optical and infrared surveys. In this work, I presented deep near-infrared adaptive optics imaging of the system, in which no source was detected at the pulsar position down to a 5-sigma limit of K_s = 21.3. Using stripped-star atmosphere models and conservative extinction estimates, I showed that any plausible helium star companion would have been detected, ruling out that interpretation. Since a massive white dwarf (WD) companion remained consistent with the non-detection, I considered two possible origins for the observed eclipses: (1) absorption in a wind driven by a young, hot WD, and (2) material ablated from the WD by the pulsar. I found that the former can naturally arise following Case BB mass transfer, which produces 1.2 solar mass WDs capable of sustaining winds sufficient to obscure the pulsar at GHz frequencies. On the other hand, the latter requires efficient coupling of the pulsar’s spin-down luminosity to the companion to drive the needed mass loss, which may be difficult to achieve. If the eclipse is powered by a WD wind, the system is likely observed in a short-lived phase; alternatively, if the companion is an older WD, the origin of the eclipsing material remains unclear. I showed that the apparent uniqueness of PSR J1928+1815 is consistent with a short detectability lifetime, though formation rate estimates remain uncertain.

Read more about this research project in the associated publication here.