Discovery of $\gamma$-Ray Pulsations from the Extreme-Spin-Down Millisecond Pulsar PSR J0435+3233

Abstract

Motivated by the recent discovery of PSR~J0435+3233, a millisecond pulsar with an exceptionally large period derivative of $\dot{P}=4.9\times10^{-17}\ {\rm s\,s^{-1}}$ and a high spin-down luminosity of $\dot{E}=5.89\times10^{37}\ {\rm erg\,s^{-1}}$, we analyze $\sim$17~yr of observations obtained with the \textit{Fermi} Large Area Telescope for the pulsar. We identify the cataloged source 4FGL~J0435.5+3232, located only $0.01^{\circ}$ from the radio timing position, as the $\gamma$-ray counterpart of PSR~J0435+3233. Using $\gamma$-ray events collected within the validity interval of the radio timing ephemeris, we detect its $\gamma$-ray pulsations at a $\sim6.8\sigma$ confidence level. In this interval, the phase-resolved analysis shows that the pulsed emission is concentrated predominantly within the rotational-phase of $\phi\sim0.44$--$0.69$. We also derive its $\gamma$-ray luminosity of $L_{\gamma}=6.26\times10^{32}\ {\rm erg\,s^{-1}}$, assuming a distance of 1.2~kpc and isotropic emission. This luminosity corresponds to an apparent $\gamma$-ray efficiency of $\eta_{\gamma}\sim1.1\times10^{-5}$, revealing an exceptionally low $\gamma$-ray output despite the pulsar's young-pulsar-like rotational-energy budget. Our detection establishes PSR~J0435+3233 as a $\gamma$-ray MSP, and the striking combination of its high spin-down power and low apparent $\gamma$-ray efficiency provides a new probe of particle acceleration, radiation beaming, and viewing geometry in the magnetospheres of millisecond pulsars with extreme rotational properties.