We report the discovery of 105 ms X-ray pulsations from the compact central object (CCO) in the supernova remnant Kes 79 using data acquired with the Newton X-Ray Multi-Mirror Mission (XMM-Newton). Two observations of the pulsar taken 6 days apart yield an upper limit on its spin-down rate of 7 × 10-14 s s-1 and no evidence for binary orbital motion. The implied energy loss rate is 2 × 1036 ergs s-1, the surface magnetic field strength is Bp 3 × 1012 G, and the spin-down age is τ 24 kyr. The latter exceeds the remnant's estimated age, suggesting that the pulsar was born spinning near its current period. The X-ray spectrum of PSR J1852+0040 is best characterized by a blackbody model of temperature kTBB = 0.44 ± 0.03 keV, radius RBB ≈ 0.9 km, and Lbol = 3.7 × 1033 ergs s-1 at d = 7.1 kpc. The sinusoidal light curve is modulated with a pulsed fraction of 45%, suggestive of a small hot spot on the surface of the rotating neutron star. The lack of a discernible pulsar wind nebula is consistent with an interpretation of PSR J1852+0040 as a rotation-powered pulsar whose spin-down luminosity falls below the empirical threshold for generating bright wind nebulae, c ≈ 4 × 1036 ergs s-1. The age discrepancy implies that its has always been below c, perhaps a distinguishing property of the CCOs. Alternatively, the X-ray spectrum of PSR J1852+0040 suggests a low-luminosity anomalous X-ray pulsar (AXP), but the weak inferred Bp field is incompatible with a magnetar theory of its X-ray luminosity. We cannot exclude accretion from a fallback disk. The ordinary spin parameters discovered from PSR J1852+0040 highlight the difficulty that existing theories of isolated neutron stars have in explaining the high luminosities and temperatures of CCO thermal X-ray spectra.
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