When initially formed, pulsars are "born" wil small rotation periods, on the order of 1 to 10 milliseconds, and and large slow down rates, P-dot, on the order of 10-12 seconds per second.
They then evolve initially along lines of constant slow down rate.
Eventually, their slow down rate decreases but their magnetic field also weakens, making their radio emission less powerful. When they are too faint to be detected, we say they have crossed the pulsar "death line" and have become invisible.
It is believed that future millisecond pulsars enter the "graveyard" as members of binary systems. As the companion evolves, mass and angular momentum are transfered from the companion to the pulsar, spinning it up.
Once "spun-up", the pulsar is "born again" as a millisecond pulsar.
The only problem with this picture is that it would predict that all millisecond pulsars are members of binary systems. But they aren't.... However, one of them, PSR 1957+20, nicknamed the "Black Widow" appears to be evaporating its companion away. So perhaps all single pulsars are "black widows".
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