The Rates of Type I X‐Ray Bursts from Transients Observed withRXTE: Evidence for Black Hole Event Horizons

We measure the rates of type I X-ray bursts, as a function of the bolometricluminosity, from a likely complete sample of 37 non-pulsing transients(1996-2004). Our goals are to test the burst model for neutron stars and toinvestigate whether black holes have event horizons. We find 135 type I burstsin 3.7 Ms of exposure for the neutron-star group, and the burst rate functionis generally consistent with model predictions. However, for the black holegroups (18 sources), there are no confirmed type I bursts in 6.5 Ms ofexposure, and the upper limits in the burst function are inconsistent with themodel predictions for heavy compact objects with a solid surface. There aresystematic spectral differences between the neutron-star and black-hole groups,supporting the presumption that physical differences underly the sampleclassifications. These results provide indirect evidence that black holes dohave event horizons.Comment: 21 pages, 9 figures, ApJ, in press for vol. 64