EGRET UPPER LIMITS TO THE HIGH-ENERGY GAMMA-RAY EMISSION FROM THE MILLISECOND PULSARS IN NEARBY GLOBULAR-CLUSTERS

We report upper limits to the high-energy gamma-ray emission from the millisecond pulsars (MSPs) in a number of globular clusters. The obser vations were done as part of an all-sky survey by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) during Phase I of the CGRO mission (1991 June to 1992 November) . Several theoretical models suggest that millisecond pulsars (MSPs) m ay be sources of high-energy gamma radiation emitted either as primary radiation from the pulsar magnetosphere or as secondary radiation gen erated by conversion into photons of a substantial part of the relativ istic e(+/-) pair wind expected to how from the pulsar. To date, no hi gh-energy emission has been detected from an individual MSP. However, a large number of MSPs are expected in globular cluster cores where th e formation rate of accreting binary systems is high. Model prediction s of the total number of pulsars range in the hundreds for some cluste rs. These expectations have been reinforced by recent discoveries of a substantial number of radio MSPs in several clusters; for example, 11 have been found in 47 Tucanae (Manchester et al.). The EGRET observat ions have been used to obtain upper limits for the efficiency eta of c onversion of MSP spin-down power into hard gamma rays. The upper limit s are also compared with the gamma-ray fluxes predicted from theoretic al models of pulsar wind emission (Tavani). The EGRET limits put signi ficant constraints on either the emission models or the number of puls ars in the globular clusters.