PROBLEMS OF ULTRA-HIGH-ENERGY PARTICLE-ACCELERATION IN CYGNUS-X-3 - A CRITICAL REAPPRAISAL

We reexamine the question of whether one can understand the acceleration of protons beyond 10(17) eV with a luminosity of L(p) approximately 10(39-41) ergs s-1 in Cyg X-3, as is believed to be necessary for explaining the gamma-ray observations from this source. We show that (i) it is implausible that some pulsar-type mechanism is at work and that for reasonable assumptions (ii) the accretion disk unipolar inductor model can be ruled out in this context. To estimate the maximum energy (E(m)) attainable by a (diffusive) parallel accretion shock, we derive an expression for E(m) as a function of the unknown surface magnetic field (B(s)) of the neutron star involved. It is seen then (iii) that even under most favorable assumptions, E(m) < 10(16)eV. More importantly, (iv) we confirm here Hillas's predicament that within the framework of standard magnetospheric theories there is little scope for extremely efficient diffusive acceleration at the accretion shock. However, the special circumstances under which an accretion shock may become an efficient accelerator is pointed out. Even then, our conclusion is that the observations from Cyg X-3, as they have been interpreted, cannot be applicable for Cyg X-3. It is suggested that the value of L(p) cannot be kept as a free (and arbitrarily high) parameter; it must be constrained by the available accretion power of the system; it must be sub-Eddington in order that the companion star not be destroyed catastrophically, and the system must have an astrophysically significant age. One can satisfy these requirements if it is realised that the TeV-PeV gamma-ray flux of Cyg X-3 can be generated by photomeson reactions on its strong X-ray field rather than by the conventional nuclear beam-dumping process.