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 noType I burst has been seen in the dozen or so binaries in which the accretingcompact star is too massive to be a neutron star and therefore is identified asa black hole candidate. We have carried out a global linear stability analysisof the accumulating fuel on the surface of a compact star to identify theconditions under which thermonuclear bursts are triggered. Our analysis, whichimproves on previous calculations, reproduces the gross observational trends ofbursts in neutron star systems. It further shows that, if black hole candidateshave surfaces, they would very likely exhibit instabilities similar to thosethat lead to Type I bursts on neutron stars. The lack of bursts in black holecandidates is thus significant, and indicates that these objects have eventhorizons. We discuss possible caveats to this conclusion.Comment: 11 pages, 1 figure, to appear in 1 August 2002 edition of Astrophysical Journal Letters, significant changes to the methods, results unchange