SLOWLY ROTATING PULSARS AND MAGNETIC-FIELD DECAY

Two dozen long period pulsars are separated from the swarm of ordinary pulsars by an obvious gap in the P versus Sd diagram (where Sd = log P + 21.0), with a plausible upper boundary for ordinary pulsars. Possi ble pulsar evolutionary tracks are discussed to explain the diagram in terms of previously suggested scenarios of magnetic field decay. The (P-Sd) diagram is difficult to understand if there is no magnetic fiel d decay during the active life of pulsars. However, if the magnetic fi elds of neutron stars decay exponentially, almost all slowly rotating pulsars must have been injected with a very long initial spin period o f about 2 seconds, which seems impossible. Based on qualitative analys es, it is concluded that magnetic fields of neutron stars decay as a p ower-law, with a time scale related to the initial field strengths. Th e plausible boundary and the gap are suggested to naturally divide pul sars with distinct magnetic ''genes'', ie. pulsars which were born fro m strongly magnetized progenitors - such as Bp stars, and pulsars born from normal massive stars. The possibility remains open that a fracti on of slowly rotating pulsars were injected with long initial spin per iods, while others would have a classical pulsar evolution history. It is suggested that PSR B1849+00 was born in the supernova remnant Kes- 79 with an initial period of about 2 seconds.