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A Search for Hard X-Ray Emission from Globular Clusters - Constraints from BATSE | | E. Ford
; P. Kaaret
; B.A. Harmon
; M. Tavani
; S.N. Zhang
; | | Date: |
23 Feb 1996 | | Subject: | astro-ph | | Affiliation: | Columbia University), P. Kaaret (Columbia University), B.A. Harmon (NASA/Marshall Space Flight Center), M. Tavani (Columbia University), and S.N. Zhang (University Space Research Association/MSFC | | Abstract: | We have monitored a sample of 27 nearby globular clusters in the hard X-ray band (20-120 keV) for approximately 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 X-ray 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 >10^36 erg s^-1 (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*10^34 erg s^-1 (2 sigma, 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) producing gamma-rays in 47 Tuc, compared to the 11 observed radio MSPs in this cluster. | | Source: | arXiv, astro-ph/9602129 | | Services: | Forum | Review | PDF | Favorites | |
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