OBSERVATION OF PULSED HARD X-RAYS GAMMA-RAYS FROM PSR 1509-58

We observed a young rotation-powered pulsar, PSR 1509 - 58, in the hard X-ray/gamma-ray or the soft gamma-ray band with a balloon-borne detector in Brazil on 1991 November 19 (UT). With a timing analysis we detected pulsations in the energy band 94-240 keV at the 150.687 ms period determined from radio observations. The pulsating flux is (7.1 +/- 1.7) x 10(-4) cm-2 s-1 in this band, and the energy spectrum follows a power law with photon index alpha = 1.64 +/- 0.4. The averaged pulse profile shows a broad single peak with a sharp rise and has a duty cycle around 50% or higher: these features are similar to what have been observed in the X-ray band by the Ginga satellite. Based on the data available now, the fraction of energy transformed from rotational energy loss to pulsed/nonpulsed soft gamma-ray radiation is estimated. If the solid angle swept by the pulsed beam is about the same as for the Crab pulsar (PSR 0531 + 21) and the Vela pulsar (PSR 0833 - 45), PSR 1509 - 58 turn out to be an extremely efficient pulsar, converting a large fraction of its rotational energy loss to radiation, as the outer gap model predicts. The observed pulsed spectrum, however, is strong in the soft gamma-ray band, in a sharp contrast to what has been observed in the Vela pulsar, a pulsar expected to be similar PSR 1509 - 58 in the outer gap model. The fact that the pulse profile remains broad and single-peaked in the soft gamma-ray band is also new for Crab-like pulsars. In these regards, PSR 1509 - 58 may require some alteration to the standard outer gap model or even a new model for gamma-ray emission in pulsars.