The Rates of type I X-ray Bursts from Transients Observed with RXTE:   Evidence for Black Hole Event Horizons
  We measure the rates of type I X-ray bursts, as a function of the bolometric luminosity, from a likely complete sample of 37 non-pulsing transients (1996-2004). Our goals are to test the burst model for neutron stars and to investigate whether black holes have event horizons. We find 135 type I bursts in 3.7 Ms of exposure for the neutron-star group, and the burst rate function is generally consistent with model predictions. However, for the black hole groups (18 sources), there are no confirmed type I bursts in 6.5 Ms of exposure, and the upper limits in the burst function are inconsistent with the model predictions for heavy compact objects with a solid surface. There are systematic spectral differences between the neutron-star and black-hole groups, supporting the presumption that physical differences underly the sample classifications. These results provide indirect evidence that black holes do have event horizons. 
Ronald A. Remillard (1), Dacheng Lin (1), Randall L.Cooper (2), and   Ramesh Narayan (2) ((1) MIT Kavli Institute for Astrop...