A Search for Hard X-Ray Emission from Globular Clusters - Constraints from BATSE

We have monitored a sample of 27 nearby globular clusters in the hard X-ray band (20 − 120 keV) for ∼1400 days using the BATSE instrument on board the Compton Gamma-Ray Observatory. Globular clusters may contain a large number of compact objects (e.g., pulsars or X-ray binaries containing neutron stars) which can produce hard Xray emission. Our search provides a sensitive (∼50 mCrab) monitor for hard X-ray transient events on time scales of ∼ > 1 day and a means for observing persistent hard X-ray emission. We have discovered no transient events from any of the clusters and no persistent emission. Our observations include a sensitive search of four nearby clusters containing dim X-ray sources: 47 Tucanae, NGC 5139, NGC 6397, and NGC 6752. The non-detection in these clusters implies a lower limit for the recurrence time of transients of 2 to 6 years for events with luminosities ∼ > 10 erg s (20 − 120 keV) and ∼ 20 years if the sources in these clusters are taken collectively. This suggests that the dim X-ray sources in these clusters are not transients similar to Aql X-1. We also place upper limits on the persistent emission in the range (2− 10)× 10 erg s (2σ, 20− 120 keV) for these four clusters. For 47 Tuc the upper limit is more sensitive than previous measurements by a factor of 3. We find a model dependent upper limit of 19 isolated millisecond pulsars (MSPs) Columbia University, Department of Physics and Columbia Astrophysics Lab, 538 W. 120th Street, New York, NY 10027 NASA/Marshall Space Flight Center, ES 66, Huntsville, AL 35812 University Space Research Association/ MSFC, ES 66, Huntsville, AL 35812