X-ray Polarization from High Mass Binaries
Abstract
The next new astrophysical window will be the advent of measurements of X-ray polarization in the 2-10 keV energy range. This will begin in the next 5 years with the launch -- of small missions such as GEMS in the US and Polarix in Italy, and may continue with IXO in the next decade. Among other things, polarization allows for potentially -- sensitive tests of the geometry of astrophysical sources, on scales which are far too small to be imaged directly. Objects which are circularly symmetric on the sky will produce no net linear polarization, a fact which led to the discovery that Seyfert galaxies are non- spherical. There is only one source in the sky whose X-ray polarization is known, the Crab nebula. Owing to visibility constraints, it is likely that the first astronomical X-ray polarimetry observations will be of objects which have never before been observed with this technique. This motivates thorough and accurate modeling of the polarization properties of the brightest and (otherwise) best understood X-ray sources, for use as calibrators and test sources for X-ray polarimetry. The sources best suited for this are X-ray binaries, in particular those in which the dominant gas component comes from a strong stellar wind from a supergiant companion star. These 'high mass X-ray binaries' (HMXBs) are among the brightest sources in the sky, their orbital elements are relatively well understood, and their orbital variability provides a predictably changing view with respect to an important source of polarization: the strong stellar wind from the companion star. In some HMXBs the X-ray source is luminous enough to ionize the wind almost completely, so the light observed during and near eclipse, and its polarization, is dominated by electron scattering. Modeling these sources is relatively straightforward, though such models do not yet exist, and these can be considered as calibration sources for astrophysical X-ray polarimeters. More generally, spectral observations show that atomic processes dominate the HMXB X-ray spectrum at low energies, more and at higher energies close to eclipse in systems where the X-ray ionization is moderate or low. Resonance line scattering can produce polarization; other atomic processes are polarization neutral or produce unpolarized radiation. These processes are coupled to the dynamical properties of the wind, and its ionization, so models must incorporate these effects. In this sense HMXB winds resemble the warm absorbers which are seen in the Chandra and XMM-Newton spectra of many active galaxies (AGN) . The goal of the proposed research is to develop a comprehensive theory for the X-ray polarization properties of HMXBs. These will take into account the properties of the stellar wind and accretion flow: its dynamics, ionization and heating under the combined influence of the early-type companion star and the accreting compact object. Also, the non-LTE opacity properties of the wind, and the transfer of X-ray photons from the compact object through the wind. The polarization imprinted by electron scattering and resonance scattering in lines will be calculated including the polarization dependence of the differential scattering cross sections dictated by quantum mechanics. We will model the wind hydrodynamics in three-dimensions, since scattering depends on the density distribution and velocity fields. We will take the intrinsic polarization of the compact object as given by, for example, existing models for magnetic accretion in the case of accreting pulsars, and with the confidence that this can be disentangled in real data using timing and energy dependence. Our work will provide us with a tool which can also be applied to predictions of polarization in wider classes of sources.
- Publication:
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NASA ATP Proposal
- Pub Date:
- 2010
- Bibcode:
- 2010atp..prop..171K