A 700 year-old pulsar in the supernova remnant Kesteven 75

Since their discovery 30 years ago, pulsars have been understood to be neut ron stars born rotating rapidly (similar to 10-100 ms). These neutron stars are thought to be created in supernova explosions involving massive stars, which give rise to expanding supernova remnants (SNRs). With over 220 Gala ctic SNRs known and over 1200 radio pulsars detected, it is quite surprisin g that few associations between the two populations have been identified wi th any certainty. Here we report the discovery of a remarkable 0.8 s X-ray pulsar, PSR J1846-0258, associated with the supernova remnant Kes 75. With a characteristic age of only 723 yr, consistent with the age of Kes 75, PSR J1846-0258 is the youngest pulsar yet discovered and is being rapidly spun down by torques from a large magnetic dipole with a strength of similar or equal to5 x 10(13) G, just above the so-called quantum critical field. PSR J1846-0258 resides in this transitional regime where the magnetic field is hypothesized to separate the regular pulsars from the so-called magnetars. PSR J1846-0258 is evidently a rotation-powered pulsar like the Crab; howev er, its period, spin-down rate, and spin-down energy-to-X-ray luminosity co nversion efficiency are each an order of magnitude greater, likely the cons equence of its extreme magnetic field.