High Temperature Component of Cen XR-2

Abstract

THE discovery in Cen XR-2 of a high temperature component with a decreasing intensity^1,2 may be consistent with the proposed nova-like model of Chodil et al.³ discussed earlier by Manley⁴. It may have arisen as the shock from the nova expanded into a circumstellar medium of radially decreasing density. It is well known^5–7 that a shock expanding into a medium with a sufficiently fast decreasing density may accelerate, so that the temperature of the shock-heated medium will increase with distance. Thus for a shock propagating into an exponential atmosphere with T(T₀) the temperature at the density ρ(ρ₀) ρ is a numerical factor between 1 and 1.5 and γ is the adiabatic index. For a density decreasing radially according to a power law, the corresponding temperature behaviour is where T(T₀) is the temperature immediately behind the shock front at R(R₀) and the density ρ∼R^−ω. (Note that ω > 3 for it to be interesting in this context.) By evaluating the integral which governs the volume emissivity at the frequency ν of a hot plasma with a radially varying temperature, In the limit hν > >kT₀ for either circumstellar density model, it is found that the spectral index (∼ 1.2) found by Lewin et al.¹ is far too small to be consistent with these simple considerations.