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

Motivated by the recent discovery of PSR J0435+3233, a millisecond pulsar with an exceptionally large period derivative of =4.9 times10-17 s,s-1 and a high spin-down luminosity of =5.89 times10 37 erg,s-1, we analyze about 17 yr of observations obtained with the Fermi Large Area Telescope for the pulsar. We identify the cataloged source 4FGL J0435.5+3232, located only 0.01 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 about6.8 sigma confidence level. In this interval, the phase-resolved analysis shows that the pulsed emission is concentrated predominantly within the rotational-phase of phi about0.44 -- 0.69. We also derive its gamma-ray luminosity of L gamma=6.26 times10 32 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 about1.1 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.