Article

Radio emission models of Colliding-Wind Binary Systems

Proceedings of the International Astronomical Union 07/2003; DOI: 10.1051/0004-6361:20031048
Source: arXiv

ABSTRACT We present calculations of the spatial and spectral distribution of the radio emission from a wide WR+OB colliding-wind binary system based on high-resolution hydrodynamical simulations and solutions to the radiative transfer equation. We account for both thermal and synchrotron radio emission, free-free absorption in both the unshocked stellar wind envelopes and the shocked gas, synchrotron self-absorption, and the Razin effect. The applicability of these calculations to modelling radio images and spectra of colliding-wind systems is demonstrated with models of the radio emission from the wide WR+OB binary WR147. Its synchrotron spectrum follows a power-law between 5 and 15 GHz but turns down to below this at lower and higher frequencies. We find that while free-free opacity from the circum-binary stellar winds can potentially account for the low-frequency turnover, models that also include a combination of synchrotron self-absorption and Razin effect are favoured. We argue that the high-frequency turn down is a consequence of inverse-Compton cooling. We present our resulting spectra and intensity distributions, along with simulated MERLIN observations of these intensity distributions. From these we argue that the inclination of the WR147 system to the plane of the sky is low. We summarise by considering extensions of the current model that are important for models of the emission from closer colliding wind binaries, in particular the dramatically varying radio emission of WR140. Comment: 18 pages, 18 figures; Accepted by Astronomy and Astrophysics, July 8, 2003

0 Bookmarks
 · 
70 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper we present observations of $\eta$ Carinae in the 1.3 mm and 7 mm radio continuum, during the 2003.5 low excitation phase. The expected minimum in the light curves was confirmed at both wavelengths and was probably due to a decrease in the number of UV photons available to ionize the gas surrounding the binary system. At 7 mm a very well defined peak was superimposed on the declining flux density. It presented maximum amplitude in 29 June 2003 and lasted for about 10 days. We show that its origin can be free-free emission from the gas at the shock formed by wind-wind collision, which is also responsible for the observed X-ray emission. Even though the shock strength is strongly enhanced as the two stars in the binary system approach each other, during periastron passage the X-ray emission is strongly absorbed and the 7 mm observations represent the only direct evidence of this event. 

    Astronomy and Astrophysics 01/2005; · 5.08 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Identification of spatially variable hydraulic rock properties such as permeability and porosity is necessary for accurate prediction of fluid flow displacement in subsurface environments. The estimation of these properties from dynamic flow data usually involves solving a highly underdetermined nonlinear inverse problem in which a limited set of measurements is combined with prior knowledge to estimate the unknown parameters. The overwhelming number of unknowns, relative to available data, leads to many parameter combinations that explain the data equally well, but fail to predict the flow behavior in the reservoir. To improve solution non-uniqueness and numerical stability, additional information is typically incorporated into the solution procedure. One way to regularize underdetermined inverse problems is to demand certain structural properties from the solution that are usually derived from the physics of the problem. In this paper, we exploit the compact representation of spatially correlated geologic formations in sparse bases to facilitate the reconstruction of rock hydraulic property distributions from flow measurements that are nonlinearly related to unknown parameters. By formulating the solution in a compressive basis such as the wavelet or Fourier, we show that minimizing a data misfit cost function augmented with an additive or multiplicative regularization term that promotes sparse solutions, the reconstruction results can be improved. Convergence to a relevant sparse solution is adaptively carried out through an iteratively reweighting algorithm in the transform domain. We evaluate the performance of our inversion algorithm using a set of two-phase waterflooding experiments in an oil reservoir where nonlinear dynamic flow data are integrated to infer the spatial distribution of rock permeability.
    Inverse Problems 09/2010; 26(10):105016. · 1.90 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In a broad and fundamental type of 'inverse problems' in science, one infers a spatially distributed physical attribute based on observations of processes that are controlled by the spatial attribute in question. The data-generating field processes, known as 'forward processes', are usually nonlinear with respect to the spatial attribute, and are often defined non-analytically by a numerical model. The data often contain a large number of elements with significant inter-correlation. We propose a general statistical method to tackle this problem. The method is centered on a parameterization device called 'anchors' and an iterative algorithm for deriving the distribution of anchors conditional on the observed data. The algorithm draws upon techniques of importance sampling and multivariate kernel density estimation with weighted samples. Anchors are selected automatically; the selection evolves in iterations in a way that is tailored to important features of the attribute field. The method and the algorithm are general with respect to the scientific nature and technical details of the forward processes. Conceptual and technical components render the method in contrast to standard approaches that are based on regularization or optimization. Some important features of the proposed method are demonstrated by examples from the earth sciences, including groundwater flow, rainfall-runoff and seismic tomography.
    Inverse Problems 11/2011; 27(12):125011. · 1.90 Impact Factor

Full-text (2 Sources)

View
22 Downloads
Available from
May 16, 2014