X-ray Modeling of \eta\ Carinae and WR140 from SPH Simulations

Proceedings of the International Astronomical Union, 2010

The colliding wind binary (CWB) systems η Carinae and WR140 provide unique laboratories for X-ray astrophysics. Their wind-wind collisions produce hard X-rays that have been monitored extensively by several X-ray telescopes, including RXTE. To interpret these RXTE X-ray light curves, we model the wind-wind collision using 3D smoothed particle hydrodynamics (SPH) simulations. Adiabatic simulations that account for the absorption of X-rays from an assumed point source at the apex of the wind-collision shock cone by the distorted winds can closely match the observed 2-10keV RXTE light curves of both η Car and WR140. This point-source model can also explain the early recovery of η Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4 reduction in the mass loss rate of η Car. Our more recent models relax the point-source approximation and account for the spatially extended emission along the wind-wind interaction shock front. For WR140, the computed X-ray light curve again matches the RXTE observations quite well. But for η Car, a hot, post-periastron bubble leads to an emission level that does not match the extended X-ray minimum observed by RXTE. Initial results from incorporating radiative cooling and radiatively-driven wind acceleration via a new anti-gravity approach into the SPH code are also discussed.

2006

We use hydrodynamical models of the wind-collision region (WCR) in the archetype colliding-wind system WR140 to determine the spatial and spectral distribution of the radio, X-ray and gamma-ray emission from shock accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission together with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with recent reductions for O4-5 supergiants. We demonstrate that radio VLBI observations of the WCR fail to constrain the opening angle. The observed low frequency turnover at ~3 GHz in the radio emission is due to free-free absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal particles. The index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration, namely p<2. Several mechanisms are discussed that could lead to such an index. Tighter constraints on p and the nature of the shocks in WR140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than previous work. Since the high stellar photon fluxes prevent the acceleration of electrons beyond gamma > 1e5-1e6, TeV emission from CWB systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Centre (abridged).

Astronomy and Astrophysics, 2009

Hot stars are sources of X-ray emission originating in their winds. Although hydrodynamical simulations that are able to predict this X-ray emission are available, the inclusion of X-rays in stationary wind models is usually based on simplifying approximations. To improve this, we use results from time-dependent hydrodynamical simulations of the line-driven wind instability (seeded by the base perturbation) to derive the analytical approximation of X-ray emission in the stellar wind. We use this approximation in our non-LTE wind models and find that an improved inclusion of X-rays leads to a better agreement between model ionization fractions and those derived from observations. Furthermore, the slope of the LX − L relation is in better agreement with observations, however the X-ray luminosity is underestimated by a factor of three. We propose a possible solution for this discrepancy.

arXiv (Cornell University), 2015

Colliding Wolf-Rayet (WR) winds produce thermal X-ray emission widely observed by X-ray telescopes. In wide WR+O binaries, such as WR 140, the X-ray flux is tied to the orbital phase, and is a direct probe of the winds' properties. In the Galactic center, $\sim$30 WRs orbit the super massive black hole (SMBH) within $\sim$10", leading to a smorgasbord of wind-wind collisions. To model the X-ray emission of WR 140 and the Galactic center, we perform 3D hydrodynamic simulations to trace the complex gaseous flows, and then carry out 3D radiative transfer calculations to compute the variable X-ray spectra. The model WR 140 RXTE light curve matches the data well for all phases except the X-ray minimum associated with periastron, while the model spectra agree with the RXTE hardness ratio and the shape of the Suzaku observations throughout the orbit. The Galactic center model of the Chandra flux and spectral shape match well in the region r$<$3", but the model flux falls off too rapidly beyond this radius.

Mon Notic Roy Astron Soc, 2006

We use hydrodynamical models of the wind-collision region (WCR) in the archetype colliding-wind system WR 140 to determine the spatial and spectral distribution of the radio, X-ray and γ-ray emission from shock accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission at this phase in conjunction with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with the reduced estimates for O4-5 supergiants by Fullerton, Massa & Prinja (2006), and the wind momentum ratio, η = 0.02. This is independent of the opening angle deduced from radio VLBI observations of the WCR that we demonstrate fail to constrain the opening angle. We show that the turnover at ∼ 3 GHz in the radio emission is due to freefree absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal electrons. We find the spectral index of the nonthermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration (DSA), namely p < 2. Several mechanisms are discussed that could lead to such an index. Our inability to obtain fits to the radio data with p > 2 does not exclude the possibility of shock modification, but stronger evidence than currently exists is necessary for its support. Tighter constraints on p and the nature of the shocks in WR 140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than previous work. Since the high stellar photon fluxes prevent the acceleration of electrons beyond γ ∼ > 10 5 − 10 6 , TeV emission from colliding-wind binary (CWB) systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Center.

X-ray line profiles represent a new way of studying the winds of massive stars. In particular, they enable us to probe in detail the wind-wind collision in colliding wind binaries, providing new insights into the structure and dynamics of the X-ray-emitting regions. We present the key results of new analyses of high-resolution Chandra X-ray spectra of two important colliding wind systems, Gamma Velorum and WR140. The lines of Gamma Vel are essentially unshifted from their rest wavelengths, which we suggest is evidence of a wide shock opening angle, indicative of sudden radiative braking. The widths of the lines of WR140 are correlated with ionization potential, implying non-equilibrium ionization. The implications of these results for the radio emission from these systems are discussed, as are some of the future directions for X-ray line profile modelling of colliding wind binaries.

Monthly Notices of the Royal Astronomical Society, 2009

The X-ray emission from the super-massive star η Car is simulated using a three dimensional model of the wind-wind collision. In the model the intrinsic X-ray emission is spatially extended and energy dependent. Absorption due to the unshocked stellar winds and the cooled postshock material from the primary LBV star is calculated as the intrinsic emission is ray-traced along multiple sightlines through the 3D spiral structure of the circumstellar environment. The observable emission is then compared to available X-ray data, including the lightcurve observed by the Rossi X-ray Timing Explorer (RXTE) and spectra observed by XMM-Newton. The orientation and eccentricity of the orbit are explored, as are the wind parameters of the stars and the nature and physics of their close approach. Our modelling supports a viewing angle with an inclination of ≃ 42 • , consistent with the polar axis of the Homunculus nebula , and the projection of the observer's line-of-sight onto the orbital plane has an angle of ≃ 0 − 30 • in the prograde direction on the apastron side of the semi-major axis.

We present calculations of the spatial and spectral distribution of the radio, X-ray and γ-ray emission from shock accelerated electrons in the wind-collision region (WCR) of WR140. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission at this phase in conjunction with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with the reduced estimates for O4-5 supergiants by Fullerton, , and the wind momentum ratio, η = 0.02.

Monthly Notices of the Royal Astronomical Society, 2006

We use hydrodynamical models of the wind-collision region in the archetype colliding-wind system WR 140 to determine the spatial and spectral distributions of the radio, X-ray, and γ -ray emission from shock-accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission at this phase in conjunction with the radio emission to constrain the mass-loss rates, we find that the O star mass-loss rate is consistent with the reduced estimates for O4-5 supergiants by Fullerton, Massa & Prinja, and the wind-momentum ratio, η = 0.02. This is independent of the opening angle deduced from radio very long baseline interferometry observations of the WCR that we demonstrate fail to constrain the opening angle.

Monthly Notices of the Royal Astronomical Society

We investigate the structure and X-ray emission from the colliding stellar winds in massive star binaries. We find that the opening angle of the contact discontinuity (CD) is overestimated by several formulae in the literature at very small values of the wind momentum ratio, η. We find also that the shocks in the primary (dominant) and secondary winds flare by ≈20 • compared to the CD, and that the entire secondary wind is shocked when η 0.02. Analytical expressions for the opening angles of the shocks, and the fraction of each wind that is shocked, are provided. We find that the X-ray luminosity L x ∝ η, and that the spectrum softens slightly as η decreases.