X-Ray Bursts in Neutron Star and Black Hole Binaries from Unconventional Stellar Aspect Experiment and Rossi X-Ray Timing Explorer Data: Detections and Upper Limits

Narayan & Heyl have developed a theoretical framework to convert suitable upper limits on type I X-ray bursts from accreting black hole candidates (BHCs) into evidence for an event horizon. However, no appropriate observational limit exists in the literature. In this paper we survey 2101.2 ks of data from the Unconventional Stellar Aspect (USA) X-ray timing experiment and 5142 ks of data from the Rossi X-Ray Timing Explorer (RXTE) experiment to obtain a formal constraint of this type. We find that 1122 ks of neutron star data yield a population-averaged mean burst rate of (1.7+/-0.4)×10^-5 bursts s^-1, while 6081 ks of BHC data yield a 95% confidence level upper limit of 4.9×10^-7 bursts s^-1. This is the first published limit of this type for BHCs. Applying the theoretical framework of Narayan & Heyl, we calculate regions of unstable luminosity, where the neutron stars are expected to burst and the BHCs would be expected to burst if they had a surface. In this unstable luminosity region, 464 ks of neutron star data yield an averaged mean burst rate of (4.1+/-0.8)×10^-5 bursts s^-1, and 1512 ks of BHC data yield a 95% confidence level upper limit of 2.0×10^-6 bursts s^-1 and a strong limit that BHCs do not burst with a rate similar to the rate of neutron stars in these unstable regions. This gives further evidence that BHCs do not have surfaces unless there is some new physics occurring on their surface.