We present an updated phase-coherent timing solution for the young , energetic pulsar PSR B1509 - 58 for twenty years of data .
Using a partially phase-coherent timing analysis , we show that the second frequency derivative is changing in time implying a third frequency derivative of \stackrel { \bf { \ldots } } { \nu } = ( -9 \pm 1 ) \times 10 ^ { -32 } s ^ { -4 } .
This value is consistent with the simple power law model of pulsar rotation .