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Paper: The Physical Basis of the LX = Lbol Empirical Law for O-Star X-Rays
Volume: 465, Four Decades of Massive Star Research - A Scientific Meeting in Honor of Anthony J. Moffat
Page: 153
Authors: Owocki, S. P.; Sundqvist, J. O.; Cohen, D. H.; Gayley, K. G.
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 = M = Lbol1.7 if the shocks are radiative, or with LX = M2 = Lbol3.4 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 parameterized 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 = M0.6 = 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.
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