Deep Adaptive Optics Imaging Rules Out a Helium Star Companion to PSR J1928+1815

PSR J1928+1815 is a 10.55 ms millisecond pulsar in a 3.6 hr orbit with a massive (1.0–1.6 M ⊙ ) companion that produces extended radio eclipses. The companion, proposed to be a stripped helium star, is undetected in optical and infrared surveys. We present deep near-infrared imaging using Keck/NIRC2 with laser guide star adaptive optics. No source is detected at the pulsar position down to a 5 σ limit of K s ≈ 21.3. Using stripped-star atmosphere models and conservative extinction estimates, we show that any plausible helium star companion would have been detected, ruling out this interpretation. A massive white dwarf (WD) companion remains consistent with the non-detection. We consider two possible origins for the eclipses: (1) absorption in a wind driven by a young, hot WD, and (2) material ablated from the WD by the pulsar. The former can naturally arise following Case BB mass transfer, which produces ∼1.2 M ⊙ WDs capable of sustaining winds of M ̇ ≳ 1 0 − 12 –10 −13 M ⊙ yr −1 for ∼10 4 –10 5 yr, sufficient to obscure the pulsar at GHz frequencies. 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. The apparent uniqueness of PSR J1928+1815 is consistent with a short detectability lifetime, though formation rate estimates remain uncertain.