X‐Ray Novae, Event Horizons, and the Exponential Metric

Abstract

Recent work has shown that the spinning magnetic fields of neutron stars (NSs) can cause soft/hard spectral state switches. The spectral state switches of black hole candidates (BHCs) could be produced in the same way. Spin frequencies and magnetic field strengths are estimated for them. Spins below 100 Hz and fields above 1010 G can account for their spectral state switches and their quiescent luminosities. It also appears that large flickering amplitudes and ultrasoft spectral peaks would be expected from radiating surfaces of massive NSs. Since BHCs share most of their spectral properties with NSs and there is as yet no proof of event horizons, the possibility that they might simply be massive NSs must be considered seriously. This opens an avenue for proof by negation but requires the use of a spacetime metric that has no event horizon. The Yilmaz exponential metric used here is shown to have an innermost marginally stable orbit with radius, binding energy, and Keplerian frequency that are within a few percent of the same quantities for the Schwarzschild metric. A maximum NS mass of ~10 M☉ is found for the Yilmaz metric. Since the two metrics essentially differ only by the presence/absence of a surface for the BHCs it should at last be possible to prove or disprove the existence of event horizons.