X-ray Novae, Event Horizons and the Exponential Metric

Recent models based on General Relativity attribute important spectral features of some accreting neutron stars to their surfaces and ”gap accretion” while observations of x-ray novae indicate a surprising lack of distinguishing spectral characteristics between these neutron stars and black hole candidates. The shared spectral characteristics strongly suggest that both have surfaces without event horizons. Since many of the black hole candidates have masses known to be in excess of the 3 M⊙ Schwarzschild limit for nuclear densities, the predicted existence of black holes may well be an error of General Relativity. The 3 M⊙ limit can be removed by the Yilmaz modifications to General Relativity. The exponential metric of the Yilmaz theory has no adjustable parameters, passes the four classic weak-field tests, permits neutron stars of 10 M⊙, and has no singularities, event horizons or black holes. General features of accretion disk and surface luminosities are compared here for gravitationally compact objects in the Yilmaz and Schwarzschild metrics. The gravitational fields, characteristic orbits and spectral characteristics of neutron stars in the two metrics are found to be very similar to 2 M⊙. Above 2.0 M⊙ in the Schwarzschild metric and 2.5 M⊙ in the Yilmaz metric, neutron stars have radii smaller than that of the innermost marginally stable orbit. The spectral characteristics associated with the resulting ”gap accretion” are precisely those exhibited by the black hole candidates. Massive stars in the Yilmaz metric are found to be capable of much more efficient conversion of accretion energy to x-ray luminosity than black holes. Additional binding energy may be released as other emissions from them. Future detailed comparisons of models with and without event horizons should finally decide the question of the existence of black holes. Contrary to common belief, mere possession of mass in excess of the Schwarzschild limit does not constitute proof that a compact object is a black hole. Subject headings: Accretion, Black Hole Physics, Stars: neutron, Stars: novae, X-