ON THE SPIN-DOWN OF PSR-B1509-58

The pulsar PSR B1509 - 58 is associated with the supernova remnant G320.4 - 1.2 (MSH 15 - 52) and has the largest pulse period derivative and the second smallest characteristic age after the Crab pulsar. Although PSR B1509 - 58 is the second youngest known pulsar, it has not glitched during an 11 yr data span and shows little timing noise, permitting an accurate study of its slowdown. We have measured its braking index to be n = 2.837 +/- 0.001 and its frequency third derivative to be nu = (-1.02 +/- 0.25) x 10(-31) S-4. The third derivative is consistent with the value expected for a constant braking index and magnetic moment, although we cannot rule out models in which these quantities vary.