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 $\dot{P}=4.9\times 10^{-17}\mathrm{s}{\mathrm{s}}^{-1}$ and a high spin-down luminosity of $\dot{E}=5.89\times 10^{37}\mathrm{erg}{\mathrm{s}}^{-1}$, 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 $\sim 6.8\sigma$ confidence level. In this interval, the phase-resolved analysis shows that the pulsed emission is concentrated predominantly within the rotational-phase of $\varphi \sim 0.44$--$0.69$. We also derive its $\gamma$-ray luminosity of $L_{\gamma }=6.26\times 10^{32}\mathrm{erg}{\mathrm{s}}^{-1}$, assuming a distance of 1.2~kpc and isotropic emission. This luminosity corresponds to an apparent $\gamma$-ray efficiency of ${\eta }_{\gamma }\sim 1.1\times 10^{-5}$, 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.

6 pages, 3 figures