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M. R. Garcia, S. S. Murray, J. E. McClintock, R. Narayan (CfA)
We have used the {\em Chandra} X-ray Observatory to observe the following six black hole X-ray transients in quiescence: GRO~J0422+32 (orbital period Porb=5.1 hr), A0620--00 (Porb=7.8 hr), GS2000+25 (Porb=8.3 hr), 4U1543--47 (Porb=27.0 hr), GRO~J1655--40 (Porb=62.9 hr), GS2023+338 (Porb=155 hr). Previous to these {\em Chandra} observations there was only a single detection of a short period (Porb < 50 hr) quiescent black hole transient, that of A0620--00. To this we add the detection of two more short period black hole transients in quiescence (GRO~J0422+32 and GS2000+25), and we derive an upper limit for 4U1543--47 which is a factor of 30 lower than previous limits. We find that quiescent black hole transients are more than 100 times dimmer in X-rays than quiescent neutron star transients with similar orbital periods. We interpret this dramatic difference as evidence that black hole candidates in X-ray transients have event horizons. We conclude that black holes really are black, as predicted by theory.
This work was supported by NSF via grant AST-9820686 and NASA via Contract NAS8-39073 to the {\em Chandra} X-ray Center.