  We report the discovery of 112-ms X-ray pulsations from RX J0822-4300, the
compact central object (CCO) in the supernova remnant Puppis A, in two archival
Newton X-Ray Multi-Mirror Mission observations taken in 2001. The sinusoidal
light curve has a pulsed fraction of 11% with an abrupt 180 deg. change in
phase at 1.2 keV. The observed phase shift and modulation are likely the result
of emission from opposing thermal hot spots of distinct temperatures.
Phase-resolved spectra reveal an emission feature at E(line) = 0.8 keV
associated with the cooler region, possibly due to an electron cyclotron
resonance effect similar to that seen in the spectrum of the CCO pulsar 1E
1207.4-5209. No change in the spin period of PSR J0821-4300 is detected in 7
months, with a 2 sigma upper limit on the period derivative less than 8.3E-15.
This implies limits on the spin-down energy loss rate of less than 2.3E35
erg/s, the surface magnetic dipole field strength B_s < 9.8E11 G, and the
spin-down age tau > 220 kyr. The latter is much longer than the SNR age,
indicating that PSR J0821-4300 was born spinning near its present period. Its
properties are remarkably similar to those of the two other known CCO pulsars,
demonstrating the existence of a class of neutron stars born with weak magnetic
fields related to a slow original spin. These results are also of importance in
understanding the extreme transverse velocity of PSR J0821-4300, favoring the
hydrodynamic instability mechanism in the supernova explosion.
