DISCOVERY OF A 112 ms X-RAY PULSAR IN PUPPIS A: FURTHER EVIDENCE OF NEUTRON STARS WEAKLY MAGNETIZED AT BIRTH

We report the discovery of 112 ms X-ray pulsations from RX J0822-4300, the compact central object (CCO) in the supernova remnant (SNR) 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 degrees 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 seven months, with a 2 sigma upper limit on the period derivative of (P) over dot < 8.3 x 10(-15). This implies limits on the spin-down energy loss rate of (E) over dot < 2.3 x 10(35) erg s(-1), the surface magnetic dipole field strength B-s < 9.8 x 10(11) G, and the spin-down age tau(c) > 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.