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

Abstract

PSR J1928+1815 is a 10.55 ms millisecond pulsar in a 3.6 hr orbit with a massive ($1.0$-$1.6M_{\odot }$) 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 $5\sigma$ limit of $K_s\approx 21.3$. 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 $\sim 1.2M_{\odot }$ WDs capable of sustaining winds of $\dot{M}\gtrsim 10^{-12}$-$10^{-13}{M}_{\odot }{\mathrm{yr}}^{-1}$ for $\sim 10^4$-$10^5$ 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.

14 pages, 6 figures, Accepted to PASP