Results for "Kinwah Wu"

total 11992took 0.11s
Properties of discrete X-ray sources in the starburst spiral galaxy M83Jul 10 2003We have identified 127 discrete sources in a Chandra observation of M83, with a detection limit of ~ 3 x 10^{36} erg/s in the 0.3-8.0 keV band. We discuss the individual properties of about 20 bright sources with L_x >~ 10^{38} erg/s, and the statistical ... More
Spatial dispersion of light rays propagating through a plasma in Kerr spacetimeJan 11 2019Feb 05 2019We investigate the propagation of light through a plasma on a background Kerr spacetime via a Hamiltonian formulation. The behaviour of light when propagating through a vacuum and through a plasma is not the same; the convolution of gravitational and ... More
Dark halo microphysics and massive black hole scaling relations in galaxiesSep 23 2014We 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 ... More
Molecular Line Profiles of Collapsing Gas CloudsAug 07 2009Emission line profiles of tracer molecule H$_2$CO 140 GHz transition from gravitational core collapsing clouds in the dynamic process of forming protostars are calculated, using a simple ray-tracing radiative transfer model. Three self-similar dynamic ... More
Polarization of Compton X-rays from Jets in AGNJun 11 2009We 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 ... More
Ionisation Structure in Accretion Shocks with a Composite Cooling FunctionJun 04 2001We have investigated the ionisation structure of the post-shock regions of magnetic cataclysmic variables using an analytic density and temperature structure model in which effects due to bremsstrahlung and cyclotron cooling are considered. We find that ... More
State transitions in LMC X-3Feb 01 2002We carried out a multiwavelenght study of the black-hole candidate LMC X-3 with XMM-Newton. The system showed a transition to a low-hard state, in which the X-ray spectrum was well fitted by a simple power law. It then returned to a high-soft state, characterised ... More
On the Nature of Ultra-Luminous X-ray Sources from Optical/IR MeasurementsApr 11 2007We present a model for the prediction of the optical/infra-red emission from ULXs. In the model, ULXs are binary systems with accretion taking place through Roche lobe overflow. We show that irradiation effects and presence of an accretion disk significantly ... More
Optical Spectroscopy of GX339-4 during the High-Soft and Low-Hard States II: Line Ionisation and Emission RegionAug 01 2000We have carried out observations of the X-ray transient GX339-4 during its high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H-alpha line has a single-peaked profile in the low-hard ... More
Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24Aug 05 1999We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I band light curve is modulated on the 9.5 min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J. ... More
Measuring the Motion of the Black Hole in GRO J16550-40Jan 13 1998We report optical spectroscopic observations of the Low Mass X-ray Binary GRO J1655-40 during outbursts in August-September 1994 and in June 1996. The spectra show emission and absorption lines from the accretion disk surrounding the X-ray source, which ... More
Magnetic Interaction in Ultra-compact Binary SystemsJun 11 2009This article reviews the current works on ultra-compact double-degenerate binaries in the presence of magnetic interaction, in particular, unipolar induction. The orbital dynamics and evolution of compact white-dwarf pairs are discussed in detail. Models ... More
Populations of X-ray binaries and the dynamical history of their host galaxiesMar 11 2001Sep 13 2001The observed luminosity distributions of X-ray sources indicate the presence of several populations of X-ray binaries in the nearby galaxies. Each population has its formation and evolutionary history, depending on the host environment. The features seen ... More
SPMHD simulations of Structure FormationNov 21 2018The intracluster medium of galaxy clusters is permeated by {\mu}G magnetic fields. Observations with current and future facilities have the potential to illuminate the role of these magnetic fields play in the astrophysical processes of galaxy clusters. ... More
Time-dependent radio emission from evolving jetsMar 31 2010We 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 ... More
Finding Periods in High Mass X-Ray BinariesFeb 09 2007This is a call for amateur astronomers who have the equipment and experience for producing high quality photometry to contribute to a program of finding periods in the optical light curves of high mass X-ray binaries (HMXB). HMXBs are binary stars in ... More
Accretion in dipole magnetic fields: flow structure and X-ray emission of accreting white dwarfsApr 04 2005Field-channelled accretion flows occur in a variety of astrophysical objects, including T Tauri stars,magnetic cataclysmic variables and X-ray pulsars. We consider a curvilinear coordinate system and derive a general hydrodynamic formulation for accretion ... More
Multi-wavelength behaviour of IIIZw2Jan 31 2002IIIZw2 was observed with XMM-Newton in July 2000. Its X-ray spectrum can be described by a powerlaw of photon index Gamma = 1.7 with a Gaussian line at 6.7 keV. There is no significant evidence of intrinsic absorption within the source or of a soft X-ray ... More
Properties of the Chandra Sources in M81Dec 18 2000The 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 8.3x8.3 arcmin field of M81 are examined. Roughly half the sources ... More
The radio flaring behaviour of GRO J1655-40: an analogy with extragalactic radio sources?Feb 25 2003At radio frequencies, the current evidence for the microquasar-quasar connection is based on imaging observations showing that relativistic outflows/jets are found in both classes of objects. Some microquasars also display superluminal motion, further ... More
Chandra observations of the globular cluster M54Oct 07 2005We have carried out a Chandra observation of the globular cluster M54. We detected 7 sources located within the half-mass radius of M54, at a flux limit of 1.5x10^-15 erg/s/cm in the 0.3-8 keV energy band. The spatial distribution and the colour/spectral ... More
X-ray sources in the starburst spiral galaxy M83: nuclear region and discrete source populationJan 04 2002Chandra has resolved the starburst nuclear region of the face-on grand-design spiral M83. Eighty-one point sources are detected above 3.5 sigma in the ACIS S3 image, and 15 of them are within the inner 16-arcsec region of the galaxy. A point source with ... More
Covariant Compton Scattering Kernel in General Relativistic Radiative TransferMay 26 2013A 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 ... More
Discrete and diffuse X-ray emission in the nucleus and disk of the starburst spiral galaxy M83Feb 01 2002We have studied the face-on, barred spiral M83 (NGC 5236) with Chandra. Eighty-one point sources are detected (above 3.5-sigma) in the ACIS S3 image: 15 of them are within the inner 16 arcsec region (starburst nucleus, resolved for the first time with ... More
Variations in emission from episodic plasmoid ejecta around black holesOct 06 2015The X-ray and radio flares observed in X-ray binaries and active galactic nuclei (AGN) are attributed to energetic electrons in the plasma ejecta from the accretion flows near the black hole in these systems. It is argued that magnetic reconnection could ... More
The lower boundary of the accretion column in magnetic cataclysmic variablesMay 09 2001Using a parameterised function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with ... More
Radial structure, inflow & central mass of stationary radiative galaxy clustersSep 22 2008We analyse the radial structure of self-gravitating spheres consisting of multiple interpenetrating fluids, such as the X-ray emitting gas and the dark halo of a galaxy cluster. In these dipolytropic models, adiabatic dark matter sits in equilibrium, ... More
Chandra X-ray Observations of the Spiral Galaxy M81Jun 11 2002A Chandra X-Ray Observatory ACIS-S 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 ... More
Complex orbital dynamics of a double neutron star system revolving around a massive black holeJan 14 2013We investigate the orbital dynamics of hierarchical three-body systems containing a double neutron star system orbiting around a massive black hole. These systems show complex dynamical behaviour because of relativistic coupling between orbits of the ... More
Line emission from optically thick relativistic accretion toriSep 13 2007We calculate line emission from relativistic accretion tori around Kerr black holes and investigate how the line profiles depend on the viewing inclination, spin of the central black hole, parameters describing the shape of the tori, and spatial distribution ... More
Multivariate characterization of hydrogen Balmer emission in cataclysmic variablesAug 18 2006The ratios of hydrogen Balmer emission line intensities in cataclysmic variables are signatures of the physical processes that produce them. To quantify those signatures relative to classifications of cataclysmic variable types, we applied the multivariate ... More
Stability analyses of two-temperature radiative shocks: formulation, eigenfunctions, luminosity response and boundary conditionsFeb 25 2001We present a general formulation for stability analyses of radiative shocks with multiple cooling processes, longitudinal and transverse perturbations, and unequal electron and ion temperatures. Using the accretion shocks of magnetic cataclysmic variables ... More
Gravitational and distributed heating effects of a cD galaxy on the hydrodynamical structure of its host clusterNov 07 2013Nov 27 2013We 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 ... More
Electron-cyclotron maser emission from white-dwarf pairs and white-dwarf planetary systemsFeb 28 2003By analogy to Jovian radio emissions powered by the electromagnetic interaction between Jupiter and its moons, we propose that close magnetic-nonmagnetic white-dwarf pairs and white-dwarf planetary systems are strong radio sources. A simple model is developed ... More
Radiation Transfer of Emission Lines in Curved Space-TimeJun 17 2004We present a general formulation for ray-tracing calculations in curved space-time. The formulation takes full account of relativistic effects in the photon transport and the relative motions of the emitters and the light-of-sight absorbing material. ... More
Perturbative Analysis of Two-Temperature Radiative Shocks with Multiple Cooling ProcessesSep 30 1999The structure of the hot downstream region below a radiative accretion shock, such as that of an accreting compact object, may oscillate due to a global thermal instability. The oscillatory behaviour depends on the functional forms of the cooling processes, ... More
Dark matter concentrations in galactic nuclei according to polytropic modelsJun 22 2016We calculate the radial profiles of galaxies where the nuclear region is self-gravitating, consisting of self-interacting dark matter (SIDM) with $F$ degrees of freedom. For sufficiently high density this dark matter becomes collisional, regardless of ... More
Fast Spinning Pulsars as Probes of Massive Black Holes' GravityMar 27 2014Dwarf 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 ... More
X-ray Polarization in Relativistic JetsFeb 10 2009We investigate the polarization properties of Comptonized X-rays from relativistic jets in Active Galactic Nuclei (AGN) using Monte Carlo simulations. We consider three scenarios commonly proposed for the observed X-ray emission in AGN: Compton scattering ... More
Structures, oscillations and solitons in large-scale multi-component self-gravitating systemsNov 13 2008We investigate the structure of dynamics of large self-gravitating astrophysical systems using a self-interacting two-component model. We consider two cases, galaxy clusters and cosmic walls, for illustrations. In both cases stability analyses are conducted ... More
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic VariablesMar 03 2008Compton scattering within the accretion column of magnetic cataclysmic variables (mCVs) can induce a net polarization in the X-ray emission. We investigate this process using Monte Carlo simulations and find that significant polarization can arise as ... More
X-ray polarization from accreting white dwarfs and associated systemsJun 11 2009We 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 ... More
Optical Spectroscopy of GRO J1655-40Nov 16 1999We have obtained optical spectra of the soft X-ray transient GRO J1655-40 during different X-ray spectral states (quiescence, high-soft and hard outburst) between 1994 Aug and 1997 Jun. Characteristic features observed during the 1996-97 high-soft state ... More
Euclid & SKA SynergiesJan 16 2015Over the past few years two of the largest and highest fidelity experiments conceived have been approved for construction: Euclid is an ESA M-Class mission that will map three-quarters of the extra galactic sky with Hubble Space Telescope resolution optical ... More
Spin dynamics of a millisecond pulsar orbiting closely around a massive black holeFeb 08 2019We investigate the spin dynamics of a millisecond pulsar (MSP) in a tightly bounded orbit around a massive black hole. These binaries are progenitors of the extreme-mass-ratio-inspirals (EMRIs) and intermediate-mass-ratio-inspirals (IMRIs) gravitational ... More
Radiative transfer in black hole systemsNov 13 2008We present a convariant formulation for radiative transfer in curved space time and demonstrate some applications in the black-hole systems. We calculate the emission from semi-transparent accretion tori around black holes, for opacity provided by the ... More
Characteristic temperatures and spectral appearance of ULX disksNov 15 2007A standard disk around an accreting black hole may become effectively optically-thin and scattering dominated in the inner region, for high accretion rates (as already predicted by the Shakura-Sunyaev model). Radiative emission from that region is less ... More
Cosmological Simulations using GCMHD+Dec 01 2011Radio observations of galaxy clusters show that the intra cluster medium is permeated by \mu G magnetic fields. The origin and evolution of these cosmological magnetic fields is currently not well understood and so their impact on the dynamics of structure ... More
A Study of the New X-ray Transient RXTE J2123-058 during its Post-Outburst StateJun 16 1999We carried out I, R, V and B photometric observations of the neutron star X-ray binary RXTE J2123-058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low mass binary model to interpret our observations. After folding our data on ... More
General relativistic radiative transfer: formulation and emission from structured tori around black holesJul 17 2012We 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 ... More
The Emission from Post-Shock Flows in mCVsJul 21 1999We re-examine the vertical structure of the post-shock flow in the accretion region of mCVs, and the X-ray emission as a function of height. We then predict X-ray light curves and phase-resolved spectra, taking into account the vertical structure, examine ... More
Continuum and line emission from post-shockc flowsSep 24 1998The post-shock flow in the accretion region of MCVs is considered to be a magnetically confined cooling flow. The X-ray spectrum from this hot plasma is emitted from a range of temperatures and densities. Extracting information from such spectra obtained ... More
Evidence for Superhumps in the Radio Light Curve of Algol and a New Model for Magnetic Activity in Algol SystemsOct 11 2004Extensive radio data of two Algol systems and two RS CVn binaries were re-analyzed. We found evidence for a new periodicity that we interpret as a superhump in Algol, in which it may have been expected according to its semi-detached nature and low binary ... More
Optical Spectroscopy of GX339-4 During the High-Soft and Low-Hard States (Paper I)Jun 28 1999We carried out spectroscopic observations of the candidate black-hole binary GX339-4 during its low-hard and high-soft X-ray states, in 1997 and 1998 respectively. We have found that the spectrum is dominated by emission lines of neutral elements with ... More
High resolution optical and infrared spectroscopic observations of Cir X-1Apr 09 1999Apr 12 1999We present new optical and infrared (IR) observations of Cir X-1 taken near apastron. Both sets of spectra show asymmetric emission lines. Archival optical observations show that an asymmetric H-alpha emission line has been in evidence for the past twenty ... More
Interactions between Ultra-High-Energy Particles and Protogalactic EnvironmentsAug 23 2018We investigate the interactions of energetic hadronic particles (cosmic ray protons) with photons and baryons in protogalactic environments, where the target photons are supplied by the first generations of stars to form in the galaxy and the cosmological ... More
A Magnetohydrodynamic Model for the Formation of Episodic JetsNov 18 2008Feb 24 2009Episodic ejection of plasma blobs have been observed in many black hole systems. While steady, continuous jets are believed to be associated with large-scale open magnetic fields, what causes the episodic ejection of blobs remains unclear. Here by analogy ... More
The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variablesAug 05 2008The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in stronger magnetic ... More
Photometric Observations of Three High Mass X-Ray Binaries and a Search for Variations Induced by Orbital MotionMay 10 2011We searched for long period variation in V-band, Ic-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 ... More
Hadronic Interactions of Energetic Charged Particles in Protogalactic Outflow Environments and Implications for the Early Evolution of GalaxiesJan 05 2019Feb 05 2019We investigate the interactions of energetic hadronic particles with the media in outflows from star-forming protogalaxies. These particles undergo pion-producing interactions which can drive a heating effect in the outflow, while those advected by the ... More
Atmosphere of strongly magnetized neutron stars heated by particle bombardmentNov 20 2018The magnetosphere of strongly magnetized neutron stars, such as magnetars, can sustain large electric currents. The charged particles return to the surface with large Lorentz factors, producing a particle bombardment. We investigate the transport of radiation ... More
Secular and orbital variability of Cir X-1 observed in optical spectraSep 11 2001We have observed variations in the optical emission lines from the X-ray binary Circinus X-1. These variations may be attributed both to orbital variations and to long term secular changes in line strength. We have detected double-peaked H-alpha emission ... More
Optical and Infrared Signatures of ULX'sJun 03 2005We have constructed a model to describe the optical emission from ultra-luminous X-ray sources (ULXs). We assume a binary model with a black hole accreting matter from a Roche lobe filling companion star. We consider the effects of radiative transport ... More
Effects of Gravity on the Structure of Postshock Accretion Flows in Magnetic Cataclysmic VariablesFeb 25 1999We have calculated the temperature and density structure of the hot postshock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of ... More
A Systematic Chandra study of Sgr A$^{\star}$: II. X-ray flare statisticsSep 12 2017Oct 19 2017The routinely flaring events from \sgras\ trace dynamic, high-energy processes in the immediate vicinity of the supermassive black hole. We statistically study temporal and spectral properties, as well as fluence and duration distributions, of the flares ... More
Cosmic rays in early star-forming galaxies and their effects on the interstellar mediumAug 22 2017Galaxies at high redshifts with strong star formation are sources of high-energy cosmic rays. These cosmic rays interact with the baryon and radiation fields of the galactic environment via photo-pair, photo-pion and proton-proton processes to produce ... More
A search for electron cyclotron maser emission from compact binariesSep 03 2007Unipolar induction (UI) is a fundamental physical process, which occurs when a conducting body transverses a magnetic field. It has been suggested that UI is operating in RX J0806+15 and RX J1914+24, which are believed to be ultra-compact binaries with ... More
Ultra-Short Period Double-Degenerate BinariesFeb 12 2003We review the current observational status of the ROSAT sources RX J1914.4+2456 and RX J0806.3+1527, and the evidence that these are ultra-short period (<10min) binary systems. We argue that an Intermediate Polar interpretation can be ruled out, that ... More
Chandra Discovery of Luminous Supersoft X-Ray Sources in M81Mar 27 2002A 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 ... More
Low and High Angular Momentum Accretion Flows in BHCs: Case Study of XTE J1550-564Aug 06 2001The 1998 outburst of XTE J1550-564 started with a hard X-ray spike, rising in less than a day and declining after 3-4 days; at the same time, the soft X-ray flux was rising with a longer timescale (~ 10 days). We suggest that the soft and the initial ... More
Two-temperature accretion flows in magnetic cataclysmic variables: Structures of post-shock emission regions and X-ray spectroscopyApr 12 2005We use a two-temperature hydrodynamical formulation to determine the temperature and density structures of the post-shock accretion flows in magnetic cataclysmic variables (mCVs) and calculate the corresponding X-ray spectra. The effects of two-temperature ... More
Formation of optical lines in black-hole binariesJan 22 2001The HI Balmer emission lines of black-hole binaries show double-peaked profiles during the high-soft state and the quiescent state. In the high-soft state the profiles are asymmetric with a stronger red peak, but the profiles are symmetric in the quiescent ... More
The star-forming environment of a ULX in NGC 4559: an optical studySep 23 2004We have studied the candidate optical counterparts and the stellar population in the star-forming complex around a bright ULX in NGC4559, using HST/WFPC2, XMM-Newton/OM, and ground-based data. We find that the ULX is located near a small group of OB stars. ... More
X-ray study of HLX1: intermediate-mass black hole or foreground neutron star?Aug 19 2010Aug 23 2010We 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 ... More
ROSAT/Chandra Observations of a Bright Transient in M81Oct 15 2001We 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 ... More
An electrically powered binary star?Nov 19 2001We propose a model for stellar binary systems consisting of a magnetic and a non-magnetic white-dwarf pair which is powered principally by electrical energy. In our model the luminosity is caused by resistive heating of the stellar atmospheres due to ... More
XMM-Newton Optical Monitor observations of LMC X-3Oct 26 2000We study the optical counterpart of the black-hole X-ray binary LMC X-3, by using XMM-Newton/OM observations carried out during a low-hard X-ray state. We derive a better constraint for the temperature, mass and radius of the companion star, and we show ... More
High-energy neutrino fluxes from AGN populations inferred from X-ray surveysJun 19 2015High-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 ... More
Long-term X-ray variability of Swift J1644+57Jan 25 2012Feb 13 2012We 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. ... More
Steady General Relativistic Magnetohydrodynamic Inflow/Outflow Solution along Large-Scale Magnetic Fields that Thread a Rotating Black HoleJan 09 2015Apr 18 2015General relativistic magnetohydrodynamic (GRMHD) flows along magnetic fields threading a black hole can be divided into inflow and outflow parts, according to the result of the competition between the black hole gravity and magneto-centrifugal forces ... More
An ultra-relativistic outflow from a neutron star accreting gas from a companionJan 15 2004Collimated relativistic outflows, or jets, are amongst the most energetic and relativistic phenomena in the Universe. They are associated with supermassive black holes in distant active galactic nuclei (AGN), accreting black holes and neutron stars in ... More
XMM-Newton EPIC and RGS observations of LMC X-3Nov 09 2000We report the results of preliminary analysis of the XMM_Newton EPIC and RGS observations of the candidate black-hole binary LMC X-3 between February and June 2000. The observations covered both the soft and the hard X-ray spectral states. The hard-state ... More
On the Nature of the Bright Short-Period X-ray Source in the Circinus Galaxy FieldNov 12 2003The spectrum and light curve of the bright X-ray source CG X-1 in the field of the Circinus galaxy are re-examined. Previous analyses have concluded that the source is an accreting black hole of about 50 solar masses although it was noted that the light ... More
Pre-merger electromagnetic counterparts of binary compact starsSep 29 2018Nov 20 2018We investigate emission signatures of binary compact star gravitational wave sources consisting of strongly magnetized neutron stars (NSs) and/or white dwarfs (WDs) in their late-time inspiral phase. Because of electromagnetic interactions between the ... More
On the orbital properties of millisecond pulsar binariesJul 24 2018We report a detailed analysis of the orbital properties of binary millisecond pulsar (MSP) with a white dwarf (WD) companion. Positive correlations between the orbital period $P_{\rm b}$ and eccentricity $\epsilon$ are found in two classes of MSP binaries ... More
Probable intermediate mass black holes in NGC4559: XMM-Newton spectral and timing constraintsNov 12 2003We have examined X-ray and optical observations of two ultra-luminous X-ray sources, X7 and X10 in NGC4559, using XMM-Newton, Chandra and HST. The UV/X-ray luminosity of X7 exceeds 2.1e40 erg/s in the XMM-Newton observation, and that of X10 is >1.3e40 ... More
Accretion Flow Along a Dipolar Field: Application to Intermediate PolarsFeb 06 2003A hydrodynamic formulation for accretion flow channeled by a dipolar magnetic field is constructed using a curvi-linear coordinate system natural to the field structure. We solve the hydrodynamic equations and determine the velocity, density and temperature ... More
On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)Sep 23 2005We combine 9 ROSAT, 9 Chandra, and 2 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_X>10^{-12} ergs cm^{-2} ... More
X-ray emissions from two-temperature accretion flows within a dipole magnetic funnelMay 09 2007We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium (two-temperature effects) due to strong radiative ... More
XMM-Newton observations of the ultra-compact binary RX J1914+24Dec 13 2004We present XMM-Newton observations of the 569 sec period system RX J1914+24 (V407 Vul). This period is believed to represent the binary orbital period making it an ultra-compact binary system. By comparing the phase of the rise to maximum X-ray flux at ... More
XMM-Newton observations of polars in low accretion statesFeb 23 2004We have made a series of snap-shot observations of 37 polars using XMM-Newton. We found that 16 of these systems were in a low, or much reduced, accretion state. Of those, 6 were not detected in X-rays. This suggests that in any survey of polars, around ... More
The Ultra-Luminous X-ray Source Population from the Chandra Archive of GalaxiesMay 25 2004May 25 2004One hundred fifty-four discrete non-nuclear Ultra-Luminous X-ray (ULX) sources, with intrinsic X-ray luminosities >1e39 ergs/s, are identified in 82 galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, ... More
Spin up in RX J0806+15 - the shortest period binaryMay 15 2003RX J0806+15 has recently been identified as the binary system with the shortest known orbital period. We present a series of observations of RX J0806+15 including new optical observations taken one month apart. Using these observations and archival data ... More
Observable Emission Features of Black Hole GRMHD Jets on Event Horizon ScalesJul 21 2017The general-relativistic magnetohydrodynamical (GRMHD) formulation for black hole-powered jets naturally gives rise to a stagnation surface, wherefrom inflows and outflows along magnetic field lines that thread the black hole event horizon originate. ... More
Polarized thermal emission from X-ray Dim Isolated Neutron Stars: the case of RX J1856.5-3754Apr 06 2016The observed polarization properties of thermal radiation from isolated, cooling neutron stars depend on both the emission processes at the surface and the effects of the magnetized vacuum which surrounds the star. Here we investigate the polarized thermal ... More
Relativistic reflection X-ray spectra of accretion disksMar 05 2009We have calculated the relativistic reflection component of the X-ray spectra of accretion disks in active galactic nuclei (AGN). Our calculations have shown that the spectra can be significantly modified by the motion of the accretion flow and the gravity ... More
A Multi-wavelength Study of the X-ray Sources in NGC 5018Feb 04 2005The E3 giant elliptical galaxy NGC5018 was observed with the Chandra X-ray Observatory on 14 April 2001. Results of analysis of these X-ray data as well as of complementary optical, infrared, and radio data are reported. Seven X-ray point sources, including ... More
Outbursts of the intermediate-mass black hole HLX-1: a wind instability scenarioApr 18 2017We model the intermediate-mass black hole HLX-1, using the Hubble Space Telescope, XMM-Newton and Swift. We quantify the relative contributions of a bluer component, function of X-ray irradiation, and a redder component, constant and likely coming from ... More
Covariant polarized radiative transfer on cosmological scales for investigating large-scale magnetic field structuresJan 14 2019Polarization of radiation is a powerful tool to study cosmic magnetism and analysis of polarization can be used as a diagnostic tool for large-scale structures. In this paper, we present a solid theoretical foundation for using polarized light to investigate ... More
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356Sep 20 2006We 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 ... More