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J. E. Grindlay (CfA)
We estimate the number of stellar mass black holes (BH) vs. neutron stars (NS) in the Galaxy by re-considering the detection rates of x-ray transients. The recent and ongoing discoveries of fast (~1d) x-ray transients made with the RXTE/ASM and BeppoSAX/WFC, such as V4641 Sgr, merit a new look at this question which has previously been considered with detection rates of systems with typically much longer duration (e.g. ~30d exponential decays). The increasing sample of moderately faint (~10-50 mCrab) transients also suggests a much larger reservoir of systems than the ~1000-3000 BH transients, and comparable or smaller number containing NS, previously derived. The sensitivity and resolution requirements for the proposed deep hard x-ray survey mission EXIST, which could achieve ~1 mCrab (5-100 keV) all-sky imaging sensitivity each 95min orbit if configured as an array of 8 \times 1m2 coded aperture telescopes on the International Space Station, are considered. The ability to distinguish BH vs. NS systems by their hard x-ray spectra, luminosities, and timing characteristics allow an estimate of the relative number of transients, and (for current formation/evolution scenarios) thus stellar remnants, in the Galaxy. We consider important sources of measurement ``backgrounds”, such as CV outbursts (dwarf novae) and RS CVn flares that will contribute to the transient detection rates at low flux levels. Soft Gamma-ray Repeaters (SGRs) in Local Group galaxies will be another, highly interesting, source of such events readily distinguished and studied. This work was supported by NASA.
The author(s) of this abstract have provided an email address for comments about the abstract: josh@cfa.harvard.edu