Very high energy gamma-ray observations of PSR B1509-58 with the CANGAROO 3.8 meter telescope

The gamma-ray pulsar PSR B1509-58 and its surrounding nebulae have been observed with the Collaboration of Australia and Nippon for a Gamma-Ray Observatory in the Outback (CANGAROO) 3.8 m imaging atmospheric Cerenkov telescope. The observations were performed from 1996 to 1998 in Woomera, South Australia, under different instrumental conditions with estimated threshold energies of 4.5 (1996), 1.9 (1997), and 2.5 TeV (1998) at zenith angles of similar to 30 degrees. Although no strong evidence of 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 sigma significance level. The corresponding gamma-ray flux is calculated to be (2.9 +/- 0.7) x 10(-12) cm(-2) s(-1) above 1.9 TeV. The observations of 1996 and 1998 yielded only upper limits (99.5% confidence level) of 1.9 x 10(-12) cm(-2) s(-1) above 4.5 TeV and 2.0 x 10(-12) cm(-2) s(-1) above 2.5 TeV. 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 field strength similar or equal to 5 mu G. This is consistent with the equipartition value previously estimated in the X-ray nebula surrounding the pulsar. No significant periodicity at the 150 ms pulsar period has been found in any of the three years' data. The flux upper limits set from our observations are 1 order of magnitude below previously reported detections of pulsed TeV emission.