arXiv subjects

Kinwah Wu

Publications and source records attributed to Kinwah Wu.

At least 145 records · Page 8Linked to original sources

arXiv2005

Chandra observations of the globular cluster M54

We 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 properties of the 7 sources suggest that they are likely to be cataclysmic variables or LMXBs in the globular cluster. M54 shows the largest number of X-ray sources with luminosities greater than 10^32 ergs/s compared to other globular clusters observed using Chandra and XMM-Newton. We searched for a correlation between the number of sources above this luminosity level with globular cluster parameters. We found evidence that the number of sources peaks at a King concentration parameter c~1.7-1.9, with globular clusters which are core-collapsed or have low-c values having a smaller number of sources. We speculate on possible reasons for this.

astro-ph↗

arXiv2005

On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)

We 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} s^{-1} during a 10~day span in 1995 July. The inferred peak isotropic luminosity of the source therefore exceeded 3 10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) 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_F555W ~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST 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. If the optical source is the counterpart, then the X-ray and optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray pulsar A 0538-66.

astro-ph↗

arXiv2005

The First Chandra Field

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 upon 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 a 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 (> 10**{9} solar mass) black hole, accreting at a rate approaching its Eddington limit.

astro-ph↗

arXiv2005

Optical and Infrared Signatures of ULX's

We 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 and radiative equilibrium in the irradiated surfaces of both the star and a thin accretion disk. We have developed this model as a tool with which to positively identify the optical counterparts of ULXs, and subsequently derive parameters such as the black hole mass and the luminosity class and spectral type of the counterpart. We examine the dependence of the optical emission on these and other variables. We extend our model to examine the magnitude variation at infrared wavelengths, and we find that observations at these wavelengths may have more diagnostic power than in the optical. We apply our model to existing HST observations of the candidates for the optical counterpart of ULX X-7 in NGC 4559. All candidates could be consistent with an irradiated star alone, but we find that a number of them are too faint to fit with an irradiated star and disk together. Were one of these the optical counterpart to X-7, it would display a significant temporal variation.

astro-ph↗

arXiv2005

Two-temperature accretion flows in magnetic cataclysmic variables: Structures of post-shock emission regions and X-ray spectroscopy

We 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 flows are significant for systems with a massive white dwarf and a strong white-dwarf magnetic field. Our calculations show that two-temperature flows predict harder keV spectra than one-temperature flows for the same white-dwarf mass and magnetic field. This result is insensitive to whether the electrons and ions have equal temperature at the shock but depends on the electron-ion exchange rate, relative to the rate of radiative loss along the flow. White-dwarf masses obtained by fitting the X-ray spectra of mCVs using hydrodynamic models including the two-temperature effects will be lower than those obtained using single-temperature models. The bias is more severe for systems with a massive white dwarf.

astro-ph↗

arXiv2005

Accretion in dipole magnetic fields: flow structure and X-ray emission of accreting white dwarfs

Field-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 onto stellar objects confined by a stellar dipole magnetic field. The hydrodynamic equations are solved to determine the velocity, density and temperature profiles of the flow. We use accreting magnetic white-dwarf stars as an illustrative example of astrophysical applications. Our calculations show that the compressional heating due to the field geometry is as important as radiative cooling and gravity in determining the structure of the post-shock flow in accreting white-dwarf stars. The generalisation of the formulation to accretion flows channelled by higher-order fields and the applications to other astrophysical systems are discussed.

astro-ph↗

arXiv2005

A Multi-wavelength Study of the X-ray Sources in NGC 5018

The 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 the nucleus, were detected.If they are intrinsic to NGC5018, then all six non-nuclear sources have luminosities exceeding E+39 ergs/s in the 0.5-8.0 keV energy band; placing them in the class of Ultra-luminous X-ray sources. Comparison of X-ray source positions to archival HST/WFPC2 images reveal four of the six non-nuclear sources are spatially-coincident with bright, absolute-magnitude (V) around -8.6 mag, objects. These four objects have optical magnitudes and (V-I) colors consistent with globular clusters in NGC5018 but they also have X-ray-to-optical flux ratios consistent with background active galactic nuclei. Strong, unpolarized, radio emission has been detected from one of the optically-bright counterparts. Another optically-bright counterpart was observed to vary by one mag in optical observations taken 28 July 1997 and 04 Feb 1999. Extended X-ray emission is detected within a 15"radius of the galaxy center at a luminosity of E+40 ergs/s in the X-ray band. Its thermal X-ray spectrum (kT around 0.4 keV) and its spatial coincidence with strong H_alpha emission are consistent with a hot gas origin. The nucleus itself may be a weak X-ray source, around E+39 ergs/s, that displays a radio spectrum typical of AGN.

astro-ph↗

arXiv2004

XMM-Newton observations of the ultra-compact binary RX J1914+24

We 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 various epochs (this includes observations made using ROSAT, ASCA and Chandra) we find that the system is spinning up at a rate of 3.17+/-0.07x10^{-12} s/s. We find that the spectra softens as the X-ray flux declines towards the off-phase of the 569 sec period. Further, the spectra are best fitted by an absorbed blackbody component together with a broad emission feature around 0.59keV. This emission feature is most prominent at the peak of the on-phase. We speculate on its origin.

astro-ph↗

arXiv2004

Evidence for Superhumps in the Radio Light Curve of Algol and a New Model for Magnetic Activity in Algol Systems

Extensive 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 mass ratio. This is the first detection of the superhump phenomenon in the radio and the first observation of superhumps in Algol systems. According to our result, the accretion disk in Algol precesses in spite of its non-Keplerian nature and therefore this phenomenon is not restricted to the classical Keplerian disks in compact binaries.We propose that in Algol systems with short orbital periods, the disk is magnetically active as well as the secondary star. The magnetic field in the disk originates from amplification of the seed field in the magnetized material transferred from the secondary. The disk and stellar fields interact with each other, with reconnection of the field lines causing flares and particle acceleration. Relativistic particles are trapped in the field and directed toward the polar regions of the secondary star because of the dipole structure of its magnetic field. Our proposed model for the magnetic activity in Algol systems provides a simple explanation to the observed properties of Algol in the radio wavelengths, and to the presence of quiescent gyrosynchrotron emission near the polar region of the secondary star, where electrons are difficult to be confined if the field lines are open as in normal single magnetic stars. We propose that the superhump variation in the radio is generated by enhanced reconnection when the elongated side of the elliptic disk is the closest to the cool star. This leads to flares and enhancement in particle acceleration and is manifested as stronger gyrosynchrotron radiation.

astro-ph↗

arXiv2004

The star-forming environment of a ULX in NGC 4559: an optical study

We 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. The brightest point source in the Chandra error circle is consistent with a single blue supergiant of mass ~ 20 M_sun and age ~ 10 Myr. A few other stars are resolved inside the error circle: mostly blue and red supergiants with masses ~ 10-15 M_sun and ages ~ 20 Myr. This is consistent with the interpretation of this ULX as a black hole (BH) accreting from a high-mass donor star in its supergiant phase, via Roche-lobe overflow. The observed optical colors and the blue-to-red supergiant ratio suggest a low metal abundance: 0.2 <~ Z/Z_sun <~ 0.4 (Padua tracks), or 0.05 <~ Z/Z_sun <~ 0.2 (Geneva tracks). The age of the star-forming complex is <~ 30 Myr. H-alpha images show that this region has a ring-like appearance. We propose that it is an expanding wave of star formation, triggered by an initial density perturbation, in a region where the gas was only marginally stable to gravitational collapse. A possible trigger was the collision with a satellite dwarf galaxy, visible a few arcsec north-west of the complex, going through the gas-rich outer disk of NGC4559. The X-ray data favour a BH more massive (M > 50 M_sun) than typical Milky Way BH candidates. The optical data favour a ``young'' BH originating in the recent episode of massive star formation; however, they also rule out an association with young massive star clusters. We speculate that other mechanisms may lead to the formation of relatively massive BHs (~ 50-100 M_sun) from stellar evolution processes in low-metallicity environments, or when star formation is triggered by galactic collisions.

astro-ph↗

arXiv2004

Radiation Transfer of Emission Lines in Curved Space-Time

We 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. We apply the formulation to calculate the emission from accretion disks and tori around rotating black holes.

astro-ph↗

arXiv2004

The Ultra-Luminous X-ray Source Population from the Chandra Archive of Galaxies

One 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, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields are made. Sources above ~1e38 erg/s display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. Comparison of the cumulative X-ray luminosity functions of the ULXs to Chandra Deep Field results suggests ~25% of the sources may be background objects including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies implying the elliptical galaxy ULX population is severely compromised by background active galactic nuclei. Correlations with host galaxy properties confirm the number and total X-ray luminosity of the ULXs are associated with recent star formation and with galaxy merging and interactions. The preponderance of ULXs in star-forming galaxies as well as their similarities to less-luminous sources suggest they originate in a young but short-lived population such as the high-mass X-ray binaries with a smaller contribution (based on spectral slope) from recent supernovae. The number of ULXs in elliptical galaxies scales with host galaxy mass and can be explained most simply as the high-luminosity end of the low-mass X-ray binary population.

astro-ph↗

arXiv2004

XMM-Newton observations of polars in low accretion states

We 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 half will be in a low accretion state. We tested if there was a bias towards certain orbital periods: this is not the case. Of the 10 systems which were detected at low, but significant rates in X-rays, 8 showed significant variability in their X-ray light curves. This implies that non-uniform accretion still takes place during low accretion epochs. The bolometric luminosity of these systems is ~10^30 ergs s, two orders of magnitude less than for systems in a high accretion state. The X-ray spectra show no evidence of a distinct soft X-ray component. However, the X-ray and UV data imply that such a low temperature component exists: its temperature is low enough for its flux distribution to move outside the bandpass of the X-ray instruments.

astro-ph↗

arXiv2004

An ultra-relativistic outflow from a neutron star accreting gas from a companion

Collimated 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 binary systems and are believed to be responsible for gamma-ray bursts. The physics of these jets,however, remains something of a mystery in that their bulk velocities, compositions and energetics remain poorly determined. Here we report the discovery of an ultra-relativistic outflow from a binary accreting neutron star accreting gas within a binary system. The velocity of the outflow is comparable to the fastest-moving flows observed from active galactic nuclei, and its strength is modulated by the rate of accretion of material onto the neutron star. Shocks are energized further downstream in the flow, which are themselves moving with mildly relativistic bulk velocities and are the sites of the observed synchrotron emission from the jet. We conclude that the generation of such highly relativistic outflows does not require properties unique to black holes, such as an event horizon.

astro-ph↗

arXiv2003

On the Nature of the Bright Short-Period X-ray Source in the Circinus Galaxy Field

The 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 curve resembles that of an AM Her system. Here we show that the short period and an assumed main sequence companion constrain the mass of the companion to less than one solar mass. Further a possible eclipse seen during one of the Chandra observations and a subsequent XMM-Newton observation constrains the mass of the compact object to less than about 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 which intercepts this flux during eclipse will be driven out of thermal equilibrium and evaporate within about 1000 years. We find that the observations cannot rule out an AM Her system in the Milky Way and that such a system can account for the variations seen in the light curve.

astro-ph↗

arXiv2003

Probable intermediate mass black holes in NGC4559: XMM-Newton spectral and timing constraints

We 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 erg/s. X7 has both thermal and power-law spectral components. The characteristic temperature of the thermal component is 0.12 keV. The power-law components in the two sources both have slopes with photon index \~2.1. A timing analysis of X7 indicates a break frequency at 28 mHz in the power spectrum, while that for X10 is consistent with an unbroken power law. The luminosity of the blackbody component in the X-ray spectrum of X7 and the nature of its time variability provides evidence that this object is an intermediate mass black hole accreting at sub-Eddington rates, but other scenarios which require high advection efficiencies from a hollowed-out disk might be possible. The emission from X10 can be characterised by a single power-law. This source can be interpreted either as an intermediate mass black hole, or as a stellar-mass black hole with relativistically-beamed Comptonised emission. There are four optical counterparts in the error circle of X7. No counterparts are evident in the error circle for X10.

astro-ph↗

arXiv2003

Properties of discrete X-ray sources in the starburst spiral galaxy M83

We 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 properties of the whole sample. About 1/3 of the bright sources show X-ray spectra with a bb component at kT <~ 1 keV, plus a p-law component with Gamma ~ 2.5, typical of X-ray binaries (XRBs) in a soft state; another third have p-law spectra with Gamma ~ 1.5, consistent with XRBs in a hard state. Two bright sources show emission lines on a hard p-law continuum, and are probably XRBs surrounded by a photo-ionized nebula or stellar wind. Among the other bright sources, we also identified two SNR candidates, with optically-thin thermal plasma spectra at kT ~ 0.5 keV. The two brightest supersoft sources have T_bb ~ 70 eV and L_x ~ 10^{38} erg/s. Two candidate X-ray pulsars are detected with periods ~ 200 s. Another source corresponds to the core of a background FRII radio galaxy. The discrete sources can be divided into three groups, based on their spatial, color and luminosity distributions. The first group comprises supersoft sources with no detected emission above 1 keV. The second group consists of soft sources with little or no detected emission above 2 keV. They are strongly correlated with H-alpha emission in the spiral arms and starburst nucleus, tracing a young population. Their relative abundance depends on the current level of star-formation. Most of them are likely to be SNRs. The sources in the third group are mostly XRBs (in a soft or hard state), reaching higher X-ray luminosities than sources in the other two groups. Being a mixture of old low-mass and young high-mass systems, the whole group appears to be of intermediate age when correlated with H-alpha.

astro-ph↗

arXiv2003

Spin up in RX J0806+15 - the shortest period binary

RX 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 we find that the period of this system is decreasing over time. Our measurements imply f_dot = 6.11x10^-16 Hz/s, which is in agreement with a rate expected from the gravitational radiation for two white dwarfs orbiting at a given period. However, a smaller value of f_dot = 3.14x10^-16 Hz/s cannot be ruled out. Our result supports the idea that the 321.5 s period is the orbital period and that the system is the shortest period binary known so far and that it is one of the strongest sources of constant gravitational radiation in the sky. Furthermore, the decrease of the period strongly favours the unipolar inductor (or electric star) model rather than the accretion models.

astro-ph↗