Kinwah Wu

University College London, Londinium, England, United Kingdom

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Publications (138)504.87 Total impact

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    ABSTRACT: High-energy neutrinos and photons are complementary messengers, probing violent astrophysical processes and structural evolution of the Universe. X-ray and neutrino observations jointly constrain conditions in active galactic nuclei (AGN) jets: their baryonic and leptonic contents, and particle production efficiency. Testing two standard neutrino production models for local source Cen A, we calculate the high-energy neutrino spectra of single AGN sources and derive the flux of high-energy neutrinos expected for the current epoch. Assuming that accretion determines both X-rays and particle creation, our parametric scaling relations predict neutrino yield in various AGN classes. We derive redshift-dependent number densities of each class, from Chandra and Swift/BAT X-ray luminosity functions. We integrate the neutrino spectrum expected from the cumulative history of AGN (correcting for cosmological and source effects, e.g. jet orientation and beaming). Both emission scenarios yield neutrino fluxes well above limits set by IceCube (by ∼4–10⁶× at 1 PeV, depending on the assumed jet models for neutrino production). This implies that: (i) Cen A might not be a typical neutrino source as commonly assumed; (ii) both neutrino production models overestimate the efficiency; (iii) neutrino luminosity scales with accretion power differently among AGN classes and hence does not follow X-ray luminosity universally; (iv) some AGN are neutrino-quiet (e.g. below a power threshold for neutrino production); (v) neutrino and X-ray emission have different duty cycles (e.g. jets alternate between baryonic and leptonic flows); or (vi) some combination of the above.
    Monthly Notices of the Royal Astronomical Society 06/2015; 451(4). DOI:10.1093/mnras/stv1196 · 5.23 Impact Factor
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    ABSTRACT: We investigate the black hole (BH) scaling relation in galaxies using a model in which the galaxy halo and central BH are a self-gravitating sphere of dark matter (DM) with an isotropic, adiabatic equation of state. The equipotential where the escape velocity approaches the speed of light defines the horizon of the BH. We find that the BH mass (m•) depends on the DM entropy, when the effective thermal degrees of freedom (F) are specified. Relations between BH and galaxy properties arise naturally, with the BH mass and DM velocity dispersion following m• ∝ σF/2 (for global mean density set by external cosmogony). Imposing observationally derived constraints on F provides insight into the microphysics of DM. Given that DM velocities and stellar velocities are comparable, the empirical correlation between m• and stellar velocity dispersions σ⋆ implies that 7 ≲ F < 10. A link between m• and globular cluster properties also arises because the halo potential binds the globular cluster swarm at large radii. Interestingly, for F > 6 the dense dark envelope surrounding the BH approaches the mean density of the BH itself, while the outer halo can show a nearly uniform kpc-scale core resembling those observed in galaxies.
    Monthly Notices of the Royal Astronomical Society 12/2014; 445(4):3415-3434. DOI:10.1093/mnras/stu1984 · 5.23 Impact Factor
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    Dinesh Singh · Kinwah Wu · Gordon E. Sarty
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    ABSTRACT: Dwarf galaxies and globular clusters may contain intermediate mass black holes ($10^{3}$ to $10^{5}$ solar masses) in their cores. Estimates of ~$10^{3}$ neutron stars in the central parsec of the Galaxy and similar numbers in small elliptical galaxies and globular clusters along with an estimated high probability of ms-pulsar formation in those environments has led many workers to propose the use of ms-pulsar timing to measure the mass and spin of intermediate mass black holes. Models of pulsar motion around a rotating black hole generally assume geodesic motion of a "test" particle in the Kerr metric. These approaches account for well-known effects like de Sitter precession and the Lense-Thirring effect but they do not account for the non-linear effect of the pulsar's stress-energy tensor on the space-time metric. Here we model the motion of a pulsar near a black hole with the Mathisson-Papapetrou-Dixon (MPD) equations. Numerical integration of the MPD equations for black holes of mass 2 X $10^{6}$, $10^{5}$ and $10^{3}$ solar masses shows that the pulsar will not remain in an orbital plane with motion vertical to the plane being largest relative to the orbit's radial dimensions for the lower mass black holes. The pulsar's out of plane motion will lead to timing variations that are up to ~10 microseconds different from those predicted by planar orbit models. Such variations might be detectable in long term observations of millisecond pulsars. If pulsar signals are used to measure the mass and spin of intermediate mass black holes on the basis of dynamical models of the received pulsar signal then the out of plane motion of the pulsar should be part of that model.
    Monthly Notices of the Royal Astronomical Society 03/2014; 441(1). DOI:10.1093/mnras/stu614 · 5.23 Impact Factor
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    Curtis J. Saxton · Kinwah Wu
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    ABSTRACT: We investigate the effects of a cD galaxy's gravity and AGN heating of the host galaxy cluster. We consider a standard prescription for the hydrodynamics, with the structures determined by mass continuity, momentum and energy conservation equations in spherical symmetry. The cluster comprises a dark matter halo (DM) and ionised X-ray emitting intracluster gas (ICM), which jointly determine the gravitational potential. The cD galaxy is an additive gravitational potential component. The DM assumes a polytropic equation of state (determined by its microphysics), which could be non-radiative self-interacting particles or more exotically interacting particles. The AGN provides distributed heating, counteracting radiative cooling. Stationary density and velocity dispersion profiles are obtained by numerically integrating the hydrodynamic equations with appropriate boundary conditions. The minimum gas temperature in the cluster core is higher when a cD galaxy is present than when it is absent. The solutions also yield a point-like mass concentration exceeding a minimum mass: presumably the AGN's supermassive black hole (SMBH). Consistency with observed SMBH masses constrains the possible DM equations of state. The constraints are looser when a cD galaxy is present. Distributed (AGN) heating alters cluster global properties, and also reduces the lower limits for the central point-mass, for the preferred DM models in which the dark particles have greater heat capacity than point particles. Eluding these constraints would require dominant non-spherical or anisotropic effects (e.g. bulk rotation, non-radial streaming, asymmetric lumps, or a strong magnetic field).
    Monthly Notices of the Royal Astronomical Society 02/2014; 437(4):3750-3765. DOI:10.1093/mnras/stt2170 · 5.23 Impact Factor
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    ABSTRACT: (Abridged) We combine NUV, optical and IR imaging of the nearby starburst galaxy M82 to explore the properties of the dust both in the interstellar medium of the galaxy and the dust entrained in the superwind. The three NUV filters of Swift/UVOT enable us to probe in detail the properties of the extinction curve in the region around the 2175A bump. The NUV colour-colour diagram strongly rules out a Calzetti-type law, which can either reflect intrinsic changes in the dust properties or in the star formation history compared to starbursts well represented by such an attenuation law. We emphasize that it is mainly in the NUV region where a standard Milky-Way-type law is preferred over a Calzetti law. The age and dust distribution of the stellar populations is consistent with the scenario of an encounter with M81 in the recent 400 Myr. The radial gradients of the NUV and optical colours in the superwind region support the hypothesis that the emission in the wind cone is driven by scattering from dust grains entrained in the ejecta. The observed wavelength dependence reveals either a grain size distribution $n(a)\propto a^{-2.5}$, where $a$ is the size of the grain, or a flatter distribution with a maximum size cutoff, suggesting that only small grains are entrained in the supernovae-driven wind.
    Monthly Notices of the Royal Astronomical Society 01/2014; 440(1). DOI:10.1093/mnras/stu185 · 5.23 Impact Factor
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    ABSTRACT: X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).
    Experimental Astronomy 12/2013; 36(3):523-567. DOI:10.1007/s10686-013-9344-3 · 2.66 Impact Factor
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    Ziri Younsi · Kinwah Wu
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    ABSTRACT: A covariant scattering kernel is a core component in any self-consistent general relativistic radiative transfer formulation in scattering media. An explicit closed-form expression for a covariant Compton scattering kernel with a good dynamical energy range has unfortunately not been available thus far. Such an expression is essential to obtain numerical solutions to the general relativistic radiative transfer equations in complicated astrophysical settings where strong scattering effects are coupled with highly relativistic flows and steep gravitational gradients. Moreover, this must be performed in an efficient manner. With a self-consistent covariant approach, we have derived a closed-form expression for the Compton scattering kernel for arbitrary energy range. The scattering kernel and its angular moments are expressed in terms of hypergeometric functions, and their derivations are shown explicitly in this paper. We also evaluate the kernel and its moments numerically, assessing various techniques for their calculation. Finally, we demonstrate that our closed-form expression produces the same results as previous calculations, which employ fully numerical computation methods and are applicable only in more restrictive settings.
    Monthly Notices of the Royal Astronomical Society 05/2013; 433(2). DOI:10.1093/mnras/stt786 · 5.23 Impact Factor
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    ABSTRACT: Two-photon annihilation quanta are emitted in a pure quantum state and when detected in coincidence, the photon pairs possess orthogonal polarizations. We propose that this polarization correlation can be exploited in Positron Emission Tomography (PET), which relies crucially on accurate coincidence detection of photon pairs. In this proof of concept study, we investigate how photon polarization information can be exploited in PET imaging by developing a method to discern true coincidences using the polarization correlation of annihilation pairs. We demonstrate that the unique identification of true photon pairs with their polarization correlation can dramatically enhance overall PET image quality, especially for high emission rates, when conventional, energy- based coincidence detection methods become increasingly unreliable. Our results suggest that polarization-based coincidence detection offers new prospects for in vivo molecular imaging with next-generation PET systems.
    SPIE Medical Imaging; 03/2013
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    Ziri Younsi · Kinwah Wu · Steven V. Fuerst
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    ABSTRACT: We construct a general relativistic radiative transfer (RT) formulation, applicable to particles with or without mass in astrophysical settings. Derived from first principles, the formulation is manifestly covariant. Absorption and emission, as well as relativistic, geometrical and optical depth effects are treated self-consistently. The RT formulation can handle 3D geometrical settings and structured objects with variations and gradients in the optical depths across the objects and along the line-of-sight. The presence of mass causes the intensity variation along the particle bundle ray to be reduced by an aberration factor. We apply the formulation and demonstrate RT calculations for emission from accretion tori around rotating black holes, considering two cases: idealised optically thick tori that have a sharply defined emission boundary surface, and structured tori that allow variations in the absorption coefficient and emissivity within the tori. Intensity images and emission spectra of these tori are calculated. Geometrical effects, such as lensing-induced self-occulation and multiple-image contribution are far more significant in accretion tori than geometrically thin accretion disks. Optically thin accretion tori emission line profiles are distinguishable from the profiles of lines from optically thick accretion tori and optically thick geometrically thin accretion disks. Line profiles of optically thin accretion tori have a weaker dependence on viewing inclination angle than those of the optically thick accretion tori or accretion disks, especially at high viewing inclination angles. Limb effects are present in accretion tori with finite optical depths. Finally, in accretion flows onto relativistic compact objects, gravitationally induced line resonance can occur. This resonance occurs easily in 3D flows, but not in 2D flows, such as a thin accretion disk around a black hole.
    Astronomy and Astrophysics 07/2012; 545. DOI:10.1051/0004-6361/201219599 · 4.48 Impact Factor
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    ABSTRACT: We studied the X-ray timing and spectral variability of the X-ray source Sw J1644+57, a candidate for a tidal disruption event. We have separated the long-term trend (an initial decline followed by a plateau) from the short-term dips in the Swift light-curve. Power spectra and Lomb-Scargle periodograms hint at possible periodic modulation. By using structure function analysis, we have shown that the dips were not random but occurred preferentially at time intervals ~ [2.3, 4.5, 9] x 10^5 s and their higher-order multiples. After the plateau epoch, dipping resumed at ~ [0.7, 1.4] x 10^6 s and their multiples. We have also found that the X-ray spectrum became much softer during each of the early dip, while the spectrum outside the dips became mildly harder in its long-term evolution. We propose that the jet in the system undergoes precession and nutation, which causes the collimated core of the jet briefly to go out of our line of sight. The combined effects of precession and nutation provide a natural explanation for the peculiar patterns of the dips. We interpret the slow hardening of the baseline flux as a transition from an extended, optically thin emission region to a compact, more opaque emission core at the base of the jet.
    Monthly Notices of the Royal Astronomical Society 01/2012; 422(2). DOI:10.1111/j.1365-2966.2012.20739.x · 5.23 Impact Factor
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    ABSTRACT: Received [year] [month] [day]; accepted [year] [month] [day] Abstract We searched for long period variation in V -band, I C -band and RXTE X-ray light curves of the High Mass X-ray Binaries (HMXBs) LS 1698 / RX J1037.5−5647, HD 110432 / 1H 1249−637 and HD 161103 / RX J1744.7−2713 in an attempt to discover orbitally induced variation. Data were obtained primarily from the ASAS database and were supplemented by shorter term observations made with the 24-and 40-inch ANU telescopes and one of the robotic PROMPT telescopes. Fourier periodograms suggested the existence of long period variation in the V -band light curves of all three HMXBs, however folding the data at those periods did not reveal convincing periodic variation. At this point we cannot rule out the existence of long term V -band variation for these three sources and hints of longer term variation may be seen in the higher precision PROMPT data. Long term V -band observations, on the order of several years, taken at a frequency of at least once per week and with a precision of 0.01 mag, therefore still have a chance of revealing long term variation in these three HMXBs.
    Research in Astronomy and Astrophysics 04/2011; 11(8). DOI:10.1088/1674-4527/11/8/007 · 1.64 Impact Factor
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    ABSTRACT: The scientific investigation of Nature relies on mathematics. While mathematics itself is precise, its application to the understanding of nature always involves an approximation. The approximate nature of mathematical application ultimately limits the scientific approach to Nature, but the freedom to make appropriate approximations allows us to understand complex phenomena on the basis of a few simple principles in approaches that are limited only by imagination. Here we review and compare two mathematical models of natural phenomena in astrophysics and physiology. The astrophysical model describes the emission of polarized radiation from a magnetic cataclysmic variable; the physiological model describes growth and competition among ovarian follicles under the influence of circulating hormones. In each case, the understanding offered by the mathematical model must be verified by fitting the model to data; this is part of the usual scientific method. In the follicle model case, we show how model fits may also be used for medical diagnostic purposes.
    Third Algerian Workshop on Astronomy and Astrophysics; 01/2011
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    ABSTRACT: A long-term observing project to determine unknown orbital periods in High Mass X-ray Binaries (HMXB) has been underway since 2005. The primary search methods are to look for periodicities in optical light curves and in spectroscopically measured radial velocities. In the course of our searches, other non-orbital periodicities and variations were found that likely were caused by either stellar pulsation or changes in the configuration of circumstellar dust and gas. The more unusual observations were followed up by observations with the Canadian MOST and NASA&apos;s Spitzer space telescopes. The MOST data point to a low eccentricity orbit and a normally clump-free circumstellar environment in the HMXB LS 5039. The Spitzer data, still preliminary, do not show any variation above systematic instrumental effects at wavelengths of 3.6 and 4.5 μm again implying a normally clump-free circumstellar environment.
    10/2010; DOI:10.1063/1.3518346
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    ABSTRACT: The results of a coordinated space-based photometric and ground-based spectroscopic observing campaign on the enigmatic gamma-ray binary LS 5039 are reported. Sixteen days of observations from the MOST satellite have been combined with high-resolution optical echelle spectroscopy from the 2.3m ANU Telescope in Siding Spring, Australia. These observations were used to measure the orbital parameters of the binary and to study the properties of stellar wind from the O primary. We found that any broad-band optical photometric variability at the orbital period is below the 2 mmag level, supporting the scenario that the orbital eccentricity of the system is near the 0.24 +/- 0.08 value implied by our spectroscopy, which is lower than values previously obtained by other workers. The low amplitude optical variability also implies the component masses are at the higher end of estimates based on the primary's O6.5V((f)) spectral type with a primary mass of ~26 solar masses and a mass for the compact star of at least 1.8 solar masses. The mass loss rate from the O primary was determined to be 3.7E-7 to 4.8E-7 solar masses per year. Comment: Accepted for MNRAS 2010 September 22
    Monthly Notices of the Royal Astronomical Society 09/2010; 411(2). DOI:10.1111/j.1365-2966.2010.17757.x · 5.23 Impact Factor
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    ABSTRACT: We re-assess the XMM-Newton and Swift observations of HLX1, to examine the evidence for its identification as an intermediate-mass black hole. We show that the X-ray spectral and timing properties are equally consistent with an intermediate-mass black hole in a high state, or with a foreground neutron star with a luminosity of about a few times 10^{32} erg/s ~ 10^{-6} L_{Edd}, located at a distance of about 1.5 to 3 kpc. Contrary to previously published results, we find that the X-ray spectral change between the two XMM-Newton observations of 2004 and 2008 (going from power-law dominated to thermal dominated) is not associated with a change in the X-ray luminosity. The thermal component becomes more dominant (and hotter) during the 2009 outburst seen by Swift, but in a way that is consistent with either scenario.
    Monthly Notices of the Royal Astronomical Society 08/2010; 410(3). DOI:10.1111/j.1365-2966.2010.17572.x · 5.23 Impact Factor
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    ABSTRACT: We investigated the time-dependent radiative and dynamical properties of light supersonic jets launched into an external medium, using hydrodynamic simulations and numerical radiative transfer calculations. These involved various structural models for the ambient media, with density profiles appropriate for galactic and extragalactic systems. The radiative transfer formulation took full account of emission, absorption, re-emission, Faraday rotation and Faraday conversion explicitly. High time-resolution intensity maps were generated, frame-by-frame, to track the spatial hydrodynamical and radiative properties of the evolving jets. Intensity light curves were computed via integrating spatially over the emission maps. We apply the models to jets in active galactic nuclei (AGN). From the jet simulations and the time-dependent emission calculations we derived empirical relations for the emission intensity and size for jets at various evolutionary stages. The temporal properties of jet emission are not solely consequences of intrinsic variations in the hydrodynamics and thermal properties of the jet. They also depend on the interaction between the jet and the ambient medium. The interpretation of radio jet morphology therefore needs to take account of environmental factors. Our calculations have also shown that the environmental interactions can affect specific emitting features, such as internal shocks and hotspots. Quantification of the temporal evolution and spatial distribution of these bright features, together with the derived relations between jet size and emission, would enable us to set constraints on the hydrodynamics of AGN and the structure of the ambient medium. Comment: 16 pages, 18 figures, MNRAS in press.
    Monthly Notices of the Royal Astronomical Society 03/2010; DOI:10.1111/j.1365-2966.2010.16554.x · 5.23 Impact Factor
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    ABSTRACT: The existence of black holes of masses ~ 10^2-10^5 Msun has important implications for the formation and evolution of star clusters and supermassive black holes. One of the strongest candidates to date is the hyperluminous X-ray source HLX1, possibly located in the S0-a galaxy ESO243-49, but the lack of an identifiable optical counterpart had hampered its interpretation. Using the Magellan telescope, we have discovered an unresolved optical source with R = (23.80 +/- 0.25) mag and V = (24.5 +/- 0.3) mag within HLX1's positional error circle. This implies an average X-ray/optical flux ratio ~ 500. Taking the same distance as ESO243-49, we obtain an intrinsic brightness M_R = (-11.0 +/- 0.3) mag, comparable to that of a massive globular cluster. Alternatively, the optical source is consistent with a main-sequence M star in the Galactic halo (for example an M4.4 star at ~ 2.5 kpc). We also examined the properties of ESO243-49 by combining Swift/UVOT observations with stellar population modelling. We found that the overall emission is dominated by a ~5 Gyr old stellar population, but the UV emission at ~2000 Ang is mostly due to ongoing star-formation at a rate of ~ 0.03 Msun/yr. The UV emission is more intense (at least a 9-sigma enhancement above the mean) North East of the nucleus, in the same quadrant as HLX1. With the combined optical and X-ray measurements, we put constraints on the nature of HLX1. We rule out a foreground star and a background AGN. Two alternative scenarios are still viable. HLX1 could be an accreting intermediate-mass black hole in a star cluster, which may itself be the stripped nucleus of a dwarf galaxy that passed through ESO243-49, an event which might have caused the current episode of star formation. Or, it could be a neutron star in the Galactic halo, accreting from an M4-M5 donor star.
    Monthly Notices of the Royal Astronomical Society 10/2009; 405(2). DOI:10.1111/j.1365-2966.2010.16517.x · 5.23 Impact Factor
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    Aimee McNamara · Zdenka Kuncic · Kinwah Wu
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    ABSTRACT: We investigate the polarization of Compton scattered X-rays from relativistic jets in active galactic nuclei using Monte Carlo simulations. We consider three scenarios: scattering of photons from an accretion disk, scattering of cosmic microwave background (CMB) photons, and synchrotron self-Comptonization (SSC) within the jet. For Comptonization of thermal disk photons or CMB photons the maximum linear polarization attained is slightly over 20% at viewing angles close to 90 degrees. The value decreases with the viewing inclination. For SSC, the maximum value may exceed 80%. The angle dependence is complicated, and it varies with the photon injection sites. Our study demonstrates that X-ray polarization, in addition to multi-wavelength spectra, can distinguish certain models for emission and particle acceleration in relativistic jets. Comment: to appear in "X-ray Polarimetry: A New Window in Astrophysics", edited by R. Bellazzini, E. Costa, G. Matt and G. Tagliaferri
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    Kinwah Wu · Aimee McNamara · Zdenka Kuncic
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    ABSTRACT: We present our results of Monte-Carlo simulations of polarized Compton X-rays from magnetic cataclysmic variables, with realistic density, temperature and velocity structures in the accretion flow. Our study has shown that the X-ray linear polarization may reach about 8% for systems with high accretion rates viewed at a high viewing inclination angle. This value is roughly twice the maximum value obtained by previous studies which assumed a cold, static emission region with a uniform density. We also investigate the X-ray polarization properties of ultra-compact double-degenerate binaries for the unipolar-inductor and direct-impact accretor models. Our study has shown negligible X-ray polarization for the unipolar-induction model. However, the direct-impact accretor model may give X-ray polarization levels similar to that predicted for the magnetic cataclysmic variables. Comment: to appear in "X-ray Polarimetry: A New Window in Astrophysics", edited by R. Bellazzini, E. Costa, G. Matt and G. Tagliaferri
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    ABSTRACT: The direct observation of clumpy circumstellar material in high mass X-ray binaries (HMXBs) is proposed. We propose to observe the two HMXBs V420 Aur and LS 5039, based on our optical data obtained over the past four years. With precision time- series photometry from the post-cryo IRAC instrument we expect the following scientific return: (i) Characterization of the size and distribution of circumstellar clumpy material as derived from modeling the light curve variations produced as the clumps disappear behind the star. (ii) Characterization of the temperature of the clumps as inferred from the 3.6 - 4.5 micrometer color of the clumps disappearing behing the primary star. (iii) Serendipitous observation of clumpy accretion events produced as clumps heat up on their way to accretion on the neutron star or black hole.

Publication Stats

2k Citations
504.87 Total Impact Points

Institutions

  • 2000–2014
    • University College London
      • Department of Space and Climate Physics
      Londinium, England, United Kingdom
  • 1993–2013
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
  • 2009
    • The University of Hong Kong
      • Department of Physics
      Hong Kong, Hong Kong
  • 2006–2007
    • National Tsing Hua University
      • Department of Physics
      Hsin-chu-hsien, Taiwan, Taiwan
  • 1992
    • Technion - Israel Institute of Technology
      • Department of Physics
      H̱efa, Haifa, Israel
  • 1988–1990
    • Louisiana State University
      Baton Rouge, Louisiana, United States