A potential observational test to disentangle the companion of binary millisecond pulsar PSR J1928+1815

Abstract

Recently, the binary millisecond pulsar (MSP) PSR J1928+1815 (J1928) was reported to be experiencing regular radio eclipses. To produce the radio pulse emission and regular radio eclipses, this source was proposed to be most likely an MSP with a He star companion. However, the possibility that the companion is a white dwarf (WD) cannot be completely ruled out. Considering the uncertainty of interstellar dust extinction, it is difficult to confirm the nature of the companion star in the near-infrared and optical bands. Based on the measured mass function and orbital inclination angle inferred by the radio eclipses, the gravitational radiation can produce a period derivative of $-(6.8-9.6)\times10^{-13}~\rm s\,s^{-1}$ when the companion star of J1928 is an extremely massive WD ($1.4~M_\odot$). Therefore, its companion star can be identified as a He star with a mass higher than $1.4~M_\odot$ if the measured period derivative exceeds $-9.6\times10^{-13}~\rm s\,s^{-1}$. However, the expansion of a He star could cause a transfer of angular momentum from the orbital motion to the spin of the He star through a tidal interaction. The loss of orbital angular momentum results in a negative orbital period derivative, which produces an extra-contribution to the period derivative due to gravitational radiation. If the companion of J1928 is a $1.0-1.6~M_\odot$ He star, the predicted period derivative is in the range of $-(4.9-13.5)\times10^{-13}~\rm s\,s^{-1}$. In particular, the tidal interaction could help to confirm a He star with a mass of $1.35-1.5~M_\odot$. We also constrain the maximum evaporation efficiency of spin-down luminosity to be $0.006$ when the companion star of J1928 is a He star.