Discovery of Gamma-Ray Pulsations from the Millisecond Pulsar PSR J0435+3233

Abstract Motivated by the discovery of PSR J0435+3233, a millisecond pulsar (MSP) with an exceptionally large observed period derivative of P ̇ = 4.9 × 1 0 − 17 s s − 1 and a nominal spin-down luminosity of E ̇ nom = 5.89 × 1 0 37 erg s − 1 , we analyze ∼17.7 yr of Fermi-LAT data. We identify the cataloged source 4FGL J0435.5+3232, located only ∼0 . ° 01 from the radio timing position, as its counterpart. Using photons collected within the radio ephemeris-valid interval, we detect γ -ray pulsations at a significance of ∼6.8 σ . The weighted pulse profile shows that the pulsed emission is concentrated predominantly within the rotational-phase interval ϕ ≃ 0.44–0.69. Assuming a distance of 1.2 kpc and isotropic-equivalent emission, we derive a γ -ray luminosity of L γ = (6.26 ± 2.48) × 10 32 erg s −1 . If the observed period derivative is interpreted as intrinsic, this corresponds to an apparent γ -ray efficiency of η γ ,app ∼ 1.1 × 10 −5 , indicating remarkably low γ -ray output relative to the nominal spin-down power. A recently proposed hierarchical triple interpretation suggests that line-of-sight acceleration induced by a tertiary companion may contribute substantially to the observed period derivative. If this interpretation is correct, the apparent efficiency would not directly measure the intrinsic conversion efficiency, which therefore remains uncertain. Our detection establishes PSR J0435+3233 as a γ -ray-emitting MSP and provides an independent characterization of its pulsed spectrum and phase-resolved emission. The source offers a valuable test case for investigating the relationship between apparent spin-down properties and magnetospheric γ -ray emission, while motivating further constraints on its intrinsic rotational-energy budget, radiation beaming, and viewing geometry.