Characteristic ages

The characteristic age of a pulsar, which is derived from its present-day rate of slowdown (Section 5.10), is an unreliable indicator of its actual age since birth. Not only is it unsafe to assume that the rate of rotation at birth was much larger than at present, but also the slowdown law itself may change during the lifetime of the pulsar. Labelling a pulsar as ‘young’ is only secure for the small number actually identifiable with dateable supernovae. The prime example is the Crab Pulsar, but there is a growing list of identifications with supernova remnants (SNRs) which can be dated more or less precisely. Among these is the SNR 3C58, long known as a radio source and tentatively identified with the supernova of AD 1181.

Another opportunity for reasonably accurate dating is through measurement of the proper motion of the pulsar away from the centre of the remains of the supernova explosion, as described in the next section. There are few such examples of reliable dating; even when a pulsar with small characteristic age can be associated with an SNR it may only be possible to assign an upper limit to the age of the SNR itself. In this chapter on young pulsars we therefore use characteristic ages as the best generally available indication of youth, and tentatively consider all characteristic ages below 100 000 years as young.