Kinwah Wu

University College London, Londinium, England, United Kingdom

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Publications (15)59.95 Total impact

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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. · 2.97 Impact Factor
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    ABSTRACT: The Chandra X-Ray Observatory obtained a 50 ks observation of the central region of M81 using the ACIS-S in imaging mode. The global properties of the 97 X-ray sources detected in the inner 83 × 83 field of M81 are examined. Roughly half the sources are concentrated within the central bulge. The remainder are distributed throughout the disk, with the brightest disk sources lying preferentially along spiral arms. The average hardness ratios of both bulge and disk sources are consistent with power-law spectra of index Γ ~ 1.6, indicative of a population of X-ray binaries. A group of much softer sources is also present. The background-source-subtracted log N-log S distribution of the disk follows a power law of index ~-0.5 with no change in slope over three decades in flux. The log N-log S distribution of the bulge follows a similar shape but with a steeper slope above ~4 × 1037 ergs s-1. There is unresolved X-ray flux from the bulge with a radial profile similar to that of the bulge sources. This unresolved flux is softer than the average of the bulge sources, and extrapolating the bulge log N-log S distribution toward weaker sources can account for only 20% of the unresolved flux. No strong time variability was observed for any source with the exception of one bright, soft source.
    The Astrophysical Journal 12/2008; 549(1):L43. · 6.73 Impact Factor
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    ABSTRACT: The E3 giant elliptical galaxy NGC 5018 was observed with the Chandra X-Ray Observatory's Advanced CCD Imaging Spectrometer for 30 ks on 2001 April 14. Results of the analysis of these X-ray data as well as of complementary optical, infrared, and radio data are reported. Seven X-ray point sources, including the nucleus, were detected. If they are intrinsic to NGC 5018, then all six nonnuclear sources have luminosities exceeding 1039 ergs s-1 in the 0.5-8.0 keV energy band, placing them in the class of ultraluminous X-ray sources. Comparison of X-ray source positions to archival Hubble Space Telescope Wide Field Planetary Camera 2 (HST WFPC2) images reveals that four of the six nonnuclear sources are spatially coincident with bright, MV -8.6 mag objects. These four objects have optical magnitudes and (V - I) colors consistent with globular clusters in NGC 5018, but they also have X-ray-to-optical flux ratios consistent with background active galactic nuclei (AGNs). Strong, unpolarized radio emission has been detected from one of the optically bright counterparts. Another optically bright counterpart was observed to vary by ~1 mag in optical observations taken on 1997 July 28 and 1999 February 4. Extended X-ray emission is detected within an ~15'' radius of the galaxy center at a luminosity of ~1040 ergs s-1 in the X-ray band. Its thermal X-ray spectrum (kT ~ 0.4 keV) and its spatial coincidence with strong Hα emission are consistent with a hot gas origin. The nucleus itself may be a weak X-ray source, LX 3.5 × 1039 ergs s-1, which displays a radio spectrum typical of AGNs.
    The Astrophysical Journal 12/2008; 623(2):815. · 6.73 Impact Factor
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    ABSTRACT: Before the official first-light images, the Chandra X-Ray Observatory obtained an X-ray image of the field to which its focal plane was first exposed. We describe this historic observation and report our study of the first Chandra field. Chandra's Advanced CCD Imaging Spectrometer (ACIS) detected 15 X-ray sources, the brightest being dubbed Leon X-1 to honor the Chandra telescope scientist Leon Van Speybroeck. Based on our analysis of the X-ray data and spectroscopy at the European Southern Observatory (ESO; La Silla, Chile), we find that Leon X-1 is a type-1 (unobscured) active galactic nucleus (AGN) at redshift z = 0.3207. Leon X-1 exhibits strong Fe II emission and a broad-line Balmer decrement that is unusually flat for an AGN. Within the context of the eigenvector-1 correlation space, these properties suggest that Leon X-1 may be a massive (≥109 M☉) black hole, accreting at a rate approaching its Eddington limit.
    The Astrophysical Journal 12/2008; 637(2):682. · 6.73 Impact Factor
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    ABSTRACT: Technical progress in X-ray optics and in polarization-sensitive X-ray detectors, which our groups pioneered, enables a scientifically powerful, dedicated space mission for imaging X-ray polarimetry. This mission is sufficiently sensitive to measure X-ray (linear) polarization for a broad range of cosmic sources—primarily those involving neutron stars, stellar black holes, and supermassive black holes (active galactic nuclei). We describe the technical basis, the mission concept, and the physical and astrophysical questions such a mission would address.
    Proc SPIE 08/2008;
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    ABSTRACT: We report observations with the Chandra X-ray Observatory of the single, cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray observations of single white dwarfs, we also re-analyzed archival ROSAT data for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ 339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD 358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting the most restrictive upper limit to the X-ray luminosity from any cool white dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV thermal-bremsstrahlung spectrum. The corresponding limit to the electron density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data confirmed the non-detections reported by the original investigators. We discuss the implications of our and prior observations on models for coronal emission from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent constraints imposed by cyclotron radiation. In addition, we describe (in an appendix) a statistical methodology for detecting a source and for constraining the strength of a source, which applies even when the number of source or background events is small.
    The Astrophysical Journal 10/2006; · 6.73 Impact Factor
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    ABSTRACT: We combine 20 ROSAT, Chandra, and XMM-Newton observations of the Cen A galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU J132519.9-430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4 keV flux F(sub X) > 10(exp -12) ergs/sq cm/s during a 10 day span in 1995 July. The inferred peak isotropic luminosity of the source therefore exceeded 3 x 10(exp 39) ergs/s, which places the source in the class of ultra-luminous X-ray sources. Coherent pulsations at 13.264 Hz are detected at the 3 sigma level during a second bright episode (F(sub x) > 3 x 10(exp -13) ergs/sq cm/s) in 1999 December. The source is detected and varies significantly within three additional observations but is below the detection threshold in 7 observations. The X-ray spectrum in 1999 December is best described as a cut-off power law or a disk-blackbody (multi-colored disk). We also detect an optical source, m(sub F555W) approx. 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in Hubble images taken 195 days before the nearest X-ray observation. The optical brightness of this source is consistent with a late O or early B star at the distance of Cen A. The X-ray and optical behavior of 1RXH J132519.8-430312 is therefore similar to the transient Be/X-ray pulsar A 0538-66.
    The Astrophysical Journal 02/2005; · 6.73 Impact Factor
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    ABSTRACT: The spectrum and light curve of the bright X-ray source CG X-1 in the field of the Circinus galaxy are reexamined. Previous analyses have concluded that the source is an accreting black hole of mass > or approx. 50 solar masses although it has been noted that the light curve resembles that of an AM Herculis system. Here we show that the short period and an assumed main-sequence companion constrain the mass of the companion to less than 1 solar mass. Furthermore, a possible eclipse seen during one of the Chandra observations and a subsequent XMM-Newton observation constrain the mass of the compact object to less than 60 solar masses. If such a system lies in the Circinus galaxy, then the accreting object must either radiate anisotropically or strongly violate the Eddington limit. Even if the emission is beamed, then the companion star that intercepts this flux during eclipse will be driven out of thermal equilibrium and evaporate within approx. 10(exp 3) yr. We find that the observations cannot rule out an AM Herculis system in the Milky Way and that such a system can account for the variations seen in the light curve.
    The Astrophysical Journal 05/2004; · 6.73 Impact Factor
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    ABSTRACT: RX J0136.93510 is an unusual narrow-line Seyfert 1 galaxy (NLS1). We have detected extended (∼12 or ∼54 kpc at the source frame) soft X-ray emission in the ROSAT High Resolution Imager image, accounting for 20% of the total emission. We have also detected a highly blueshifted (7.6 keV in the source frame) Fe K a line in the ASCA Solid-State Imaging Spectrometer spectrum. This is the first detection of such a highly blueshifted emission line in a NLS1. Near-IR and far-IR studies indicate the presence of a possible starburst component in this NLS1. Physical models of the accretion and/or outflow and of the evolution of this NLS1 are discussed in the context of these results.
    The Astrophysical Journal 01/2004; 607. · 6.73 Impact Factor
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    ABSTRACT: The dlrr galaxy NGC 6822 is a distant member of the Local Group. It is a site of recent star formation, rich in HII regions and OB associations, as well as containing an older globular cluster population. We present results of a deep Chandra observation of NGC 6822. The brightest source is extended and most likely a SNR. In addition to spectral analysis of the brightest sources, we extend the luminosity function down to the 10(sup)35 erg/s range.
    02/2003;
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    ABSTRACT: A Chandra ACIS-S imaging observation of the nearby galaxy M81 (NGC 3031) reveals 9 luminous soft X-ray sources. The local environments, X-ray spectral properties, and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources. It is shown that the sample falls within expectations based on population synthesis models taken from the literature though the high observed luminosities (L~2.0e36 to \~3.0e38 erg/s in the 0.2-2.0 keV band) and equivalent blackbody temperatures T~40 to 80 eV) place the brightest detected M81 objects at the high luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies. This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems. Most of the sources can be explained as canonical \ssss, accreting white dwarfs powered by steady surface nuclear burning, with X-ray spectra well-fit by hot white dwarf local thermodynamic equilibrium atmosphere models. An exceptionally bright source is scrutinized in greater detail as its estimated bolometric luminosity, L~1.5e39 erg/s, greatly exceeds theoretical estimates for supersoft sources. This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions. Alternatively, a model in which the observed X-ray spectrum arises from an accretion disk around a blackhole of mass \~1200/sqrt(cos(i)) solar masses (viewed at an inclination angle i) cannot be excluded. Comment: 16 pages, 12 figures, accepted for publication in ApJ
    The Astrophysical Journal 03/2002; · 6.73 Impact Factor
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    ABSTRACT: A Chandra X-Ray Observatory ACIS-S (Advanced X-ray Astrophysics Facility (Chandra) CCD (Charge Coupled Device) Imaging Spectrometer) imaging observation is used to study the population of X-ray sources in the nearby Sab galaxy M81 (NGC 3031). A total of 177 sources are detected with 124 located within the D25 isophote to a limiting X-ray luminosity of 3e36 ergs/cm(exp 2)/s. Source positions, count rates, luminosities in the 0.3-8.0 keV band, limiting optical magnitudes, and potential counterpart identifications are tabulated. Spectral and timing analysis of the 36 brightest sources are reported including the low luminosity active galactic nucleus, SN 1993J, and the Einstein discovered ultraluminous X-ray source X6.
    02/2002;
  • Kinwah Wu, Allyn Tennant
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    ABSTRACT: Chandra observations allow us to obtain reliable luminosity function of X-ray binaries in the nearby galaxies. We derive an analytical expression for luminosity function of the X-ray binaries in normal galaxies and examine the processes that give rise to the features in the Log N(>S) - Log S plots. The parameters in the model are the birth rate and the characteristic lifespan of the binaries. The model successfully produces the features in the observed Log N(>S) - Log S plots in various nearby galaxies and in different galactic components of the spiral M81. By means of the model we assess the possibility of using the luminosity break in the Log N(>S) - Log S plot as a distance indicator of the host galaxies as proposed by Sarazin et al. (2001), and as a probe to the previous epochs of star formation as discussed in Wu et al. (2001). The model also predicts the presence of a population of faint, primordial X-ray binaries, which may be detectable in galaxies with low star-formation activities, such as dwarf spheroids.
    09/2001;
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    ABSTRACT: We present a 10-year X-ray light curve and the spectra of a peculiar X-ray transient in the spiral galaxy M81. The source was below the detection limit of ROSAT PSPC before 1993, but it brightened substantially in 1993, with luminosities exceeding the Eddington limit of a 1.5 solar mass compact accretor. It then faded and was not firmly detected in the ROSAT HRI and PSPC observations after 1994. The Chandra image obtained in 2000 May, however, shows an X-ray source at its position within the instrumental uncertainties. The Chandra source is coincident with a star-like object in the Digitized-Sky-Survey. A Hubble image suggests that the optical object may be extended. While these three observations could be of the same object, which may be an X-ray binary containing a black-hole candidate, the possibility that the ROSAT and Chandra sources are two different objects in a dense stellar environment cannot be ruled out. The Hubble data suggests that the optical object may be a globular cluster yet to be identified.
    02/2001;
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    ABSTRACT: We present simulated AXAF spectra of accreting white dwarfs, using parameters appropriate for magnetic cataclysmic variables. The very high spectral resolution that can be obtained with the High-Energy Transmission Grating of AXAF can resolve the keV X-ray emission lines that characterize the temperature, density and velocity profiles of the shock-heated emission regions of these systems. These simulations demonstrate that actual spectra will allow us to place constraints on the white-dwarf mass and the accretion rate of the systems. The high-resolution spectra also allow the measurement of the velocity of the accretion flow in regions close to the white-dwarf surface.
    Publications of the Astronomical Society of Australia 02/1998; · 3.12 Impact Factor