Very High-Energy Gamma-Ray Observations of PSR B1509 58 with the CANGAROO 3.8m Telescope

The gamma-ray pulsar PSR B1509 58 and its surrounding nebulae have been observed with the CANGAROO 3.8m imaging atmospheric Cerenkov telescope. The observations were performed from 1996 to 1998 in Woomera, South Australia, under di erent instrumental conditions with estimated threshold energies of 4.5 TeV (1996), 1.9 TeV (1997) and 2.5 TeV (1998) at zenith angles of 30 . Although no strong evidence of the gamma-ray emission was found, the lowest energy threshold data of 1997 showed a marginal excess of gamma-ray{like events at the 4.1 signi cance level. The corresponding gamma-ray ux is calculated to be (2:9 0:7) 10 12 cm 2 s 1 above 1.9 TeV. The observations of 1996 and 1998 yielded only upper limits (99.5% con dence level) of 1:9 10 12 cm 2 s 1 above 4.5 TeV and 2:0 10 12 cm 2 s 1 above 2.5 TeV, respectively. Assuming that the 1997 excess is due to Very High-Energy (VHE) gamma-ray emission from the pulsar nebula, our result, when combined with the X-ray observations, leads to a value of the magnetic eld strength ' 5 G. This is consistent with the equipartition value previously estimated in the X-ray nebula surrounding the pulsar. No signi cant periodicity at the 150ms pulsar period has been found in any of the three years' data. The ux upper limits set from our observations are one order of magnitude below previously reported detections of pulsed TeV emission. Subject headings: gamma-rays: observations | pulsars: individual (PSR B1509 58) | supernova remnants