On the lack of type I X-ray bursts in black hole X-ray binaries: Evidence for the event horizon?

Type I X-ray bursts are very common in neutron star X-ray binaries, but no type I burst has been seen in the dozen or so binaries in which the accreting compact star is too massive to be a neutron star and is therefore identified as a black hole candidate. We have carried out a global linear stability analysis of the accumulating fuel on the surface of a compact star to identify the conditions under which thermonuclear bursts are triggered. Our analysis, which improves on previous calculations, reproduces the gross observational trends of bursts in neutron star systems. It further shows that, if black hole candidates have surfaces, they would very likely exhibit instabilities similar to those that lead to type I bursts on neutron stars. The lack of bursts in black hole candidates is thus significant and indicates that these objects have event horizons. We discuss possible caveats to this conclusion.