We present the results of observations of the PSR B1509 - 58/MSH 15 - 52 system in X–rays ( 2 - 250 keV ) by the Rossi X–ray Timing Explorer .
The spectra of the peak of the pulsed component ( radio phase 0.17 - 0.53 ) is fit by a power law of photon index 1.36 \pm 0.01 , with no evidence of a high energy spectral break seen up to \sim 200 keV .
For the off-pulse spectral component , the spectrum from 2 - 250 keV is fit by a power law of photon index 2.215 \pm 0.005 .
An iron emission line at 6.7 keV with an equivalent width of 129 eV improves the fit , but only at a marginal significance .
Thermal bremsstrahlung and Raymond-Smith models produce much worse fits to the unpulsed data .
The lack of a high energy spectral break in the pulsed emission implies an efficiency of \geq 3 \% in the conversion of pulsar spindown energy to pulsed X–rays in the system .