A Steady, Radiative‐Shock Method for Computing X‐Ray Emission from Colliding Stellar Winds in Close, Massive‐Star Binaries

Abstract

We present a practical, efficient, semianalytic formalism for computing steady state X-ray emission from radiative shocks between colliding stellar winds in relatively close (orbital period up to order tens of days) massive-star, binary systems. Our simplified approach idealizes the individual wind flows as smooth and steady, ignoring the intrinsic instabilities and associated structure thought to occur in such flows. By also suppressing thin-shell instabilities for wind-collision radiative shocks, our steady state approach avoids the extensive structure and mixing that has thus far precluded reliable computation of X-ray emission spectra from timedependent hydrodynamical simulations of close-binary, wind-collision systems; but in ignoring the unknown physical level of such mixing, the luminosity and hardness of X-ray spectra derived here represent upper limits to what is possible for a given set of wind and binary parameters. A key feature of our approach is the separation of calculations for the small-scale shock-emission from the ram-pressure-balance model for determining the largescale, geometric form of the wind-wind interaction front. Integrating the localized shock emission over the full interaction surface and using a warm-absorber opacity to take account of attenuation by both the smooth wind and the compressed, cooled material in the interaction front, the method can predict spectra for a distant observer at any arbitrary orbital inclination and phase. We illustrate results for a sample selection of wind, stellar, and binary parameters, providing both full X-ray light curves and detailed spectra at selected orbital phases. The derived spectra typically have a broad characteristic form, and by synthetic processing with the standard XSPEC package, we demonstrate that they simply cannot be satisfactorily fitted with the usual attenuated single-or twotemperature thermal-emission models. We conclude with a summary of the advantages and limitations of our approach and outline its potential application for interpreting detailed X-ray observations from close, massivestar binary systems.

Hot stars observed by XMM-Newton

“…4. For the O+OB binaries with spectral information and defined period, log f X / f BOL ratio from Table 6. inhibition, where the radiation of one component decelerates the wind originating in its companion (Gayley et al 1997;Antokhin et al 2004). It should have more impact on short-period systems than on long-period binaries.…”

A model for the non-thermal emission of the very massive colliding-wind binary HD 93129A

“…Following the approach of Antokhin et al (2004), we assume that the flow along the WCR is laminar, neglecting any possible effect owing to mixing of the fluid streamlines. We also suppose that the velocity of the fluid at a given position of the WCR is equal to the projected tangential component of the stellar wind velocity.…”

X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2

“…If the wind material is continuously heated and cooled, a time-averaged description of the wind should contain a continuous distribution of plasma temperatures (e.g. Antokhin et al 2004). We therefore mimic such a distribution by using 4 plasma temperature components (T 1+T 2+T 3+T 4) with fixed kT of 0.2, 0.6, 1.0, and 4.0 keV, corresponding loosely to the peak emissivity of dominant spectral features likely to be present in the ACIS-S bandpass (H-like and He-like atoms of Ne, Mg, Si, S, and Fe).…”