X-ray Bursts in Neutron Star and Black Hole Binaries from USA Data: Detections and Upper Limits

Narayan and Heyl (2002) 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 2051.4 ks of data from the Unconventional Stellar Aspect (USA) X-ray timing experiment to obtain a formal constraint of this type. 1079.1 ks of neutron star data yield a population averaged mean burst rate of 1.76× 10 bursts s while 972.3 ks of BHC data yield a 95% confidence level upper limit of 3.1× 10 bursts s. Applying the theoretical framework of Narayan and Heyl (2002) 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 670 ks of neutron star data yield an averaged mean burst rate of 2.8 × 10 bursts s and 128 ks of BHC data yield a 95% confidence level upper limit of 2.3× 10 bursts s. [email protected] Current address: Laboratoire Leprince-Ringuet, Ecole Polytechnique, Palaiseau F-91128 NRC/NRL Cooperative Research Associate Current address: Department of Astrophysics, Oxford University