The Physical Basis of the Lx-Lbol Empirical Law for O-star X-rays

TL;DR

This paper develops a theoretical framework explaining the empirical linear relation between O-star X-ray luminosity and bolometric luminosity by considering shock cooling and mixing effects, bridging radiative and adiabatic shock models.

Contribution

It introduces a generalized formalism incorporating shock cooling length ratios and mixing effects to explain the observed Lx-Lbol scaling in O-stars.

Findings

The model reproduces the empirical Lx ~ Lbol relation with a mixing exponent m ~ 0.4.

It bridges radiative and adiabatic shock models through a unified formalism.

Suggests thin-shell mixing influences X-ray emission in stellar winds and colliding binaries.

Abstract

X-ray satellites since Einstein have empirically established that the X-ray luminosity from single O-stars scales linearly with bolometric luminosity, Lx ~ 10^-7 Lbol. But straightforward forms of the most favored model, in which X-rays arise from instability-generated shocks embedded in the stellar wind, predict a steeper scaling, either with mass loss rate Lx ~ Mdot ~ Lbol^1.7 if the shocks are radiative, or with Lx ~ Lx ~ Mdot^2 ~ Lbol^3.4 if they are adiabatic. We present here a generalized formalism that bridges these radiative vs. adiabatic limits in terms of the ratio of the shock cooling length to the local radius. Noting that the thin-shell instability of radiative shocks should lead to extensive mixing of hot and cool material, we then propose that the associated softening and weakening of the X-ray emission can be parametrized by the cooling length ratio raised to a power m, the "mixing exponent." For physically reasonable values m ~= 0.4, this leads to an X-ray luminosity Lx ~ Mdot^0.6 ~ Lbol that matches the empirical scaling. We conclude by noting that such thin-shell mixing may also be important for X-rays from colliding wind binaries, and that future numerical simulation studies will be needed to test this thin-shell mixing ansatz for X-ray emission.