Detection of VHE Gamma Rays from PSR1509-58

Takashi Sako

Solar Terrestrial Environment Laboratory, Nagoya University

Very high energy (VHE) gamma-ray emission from the gamma-ray pulsar PSR1509-58 is detected by using the CANGAROO 3.8m telescope. Observations have been performed in 1996 and 1997 with the threshold energy of 3.0TeV and 1.5TeV, respectively. The gamma-ray signal is obtained in the lower threshold observation with the integral flux of $(4.3\pm0.7)\times10^{-12} cm^{-2}s^{-1}$, although only the upper limit(3$\sigma$) of $1.2\times10^{-12} cm^{-2}s^{-1}$ is obtained in the '96 observation. The test for the periodicity with the pulsar rotation period was carried out and only the upper limits are obtained. While this pulsar is the second youngest pulsar, the previously estimated magnetic field strength of the pulsar nebula is much smaller than that of the Crab. A fast expansion of the remnant resulted from a powerful initial supernova explosion or a low density of the ambient interstellar matter is proposed to explain the large size of the nebula and the weak magnetic field. Our VHE results support this weak magnetic field, which leads the primary electrons to survive from a severe synchrotron energy loss. This fourth evidence of VHE gamma-ray emission from the pulsar nebula will confirm that the young spin powered pulsars are most possible acceleration site of electrons in our Galaxy.