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Compact Objects (compact + object)
Selected AbstractsCompact objects in globular clustersASTRONOMY & GEOPHYSICS, Issue 5 2007Article first published online: 20 SEP 200 No abstract is available for this article. [source] A one-way wave equation for modelling seismic waveform variations due to elastic heterogeneityGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2005D. A. Angus SUMMARY The application of a new one-way narrow-angle elastic wave equation to isotropic heterogeneous media is described. This narrow-angle finite-difference propagator should provide an efficient and accurate method of simulating primary body wave(s) passing through smoothly varying heterogeneous media. Although computationally slower than ray theory, the narrow-angle propagator can model frequency-dependent forward diffraction and scattering as well as the averaging effects due to smooth variations in medium parameters that vary on the sub-Fresnel zone level. Example waveforms are presented for the propagation of body waves in deterministic as well as stochastic heterogeneous 3-D Earth models. Extrapolation within deterministic media will highlight various familiar wave-diffraction and pulse-distortion effects associated with large-scale inhomogeneities, such as geometrical spreading, wavefront folding and creeping-wave diffraction by a compact object. Simulation within stochastic media will examine the effects of varying the correlation lengths of random heterogeneities on wave propagation. In particular, wave phenomena such as frequency-dependent forward scattering, the appearance of random caustics and the generation of seismic coda will be shown. [source] Interpretation of the 1998 outburst of the unique X-ray transient CI Camelopardalis (XTE J0421+560)MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006imon ABSTRACT We present an analysis of the 1998 outburst of the peculiar X-ray binary and X-ray transient CI Cam (XTE J0421+560). We discuss the observations in the framework of several possible models and argue that this outburst can be explained by the thermal instability of the accretion disc, analogous to the outbursts of soft X-ray transients. Applying the model by King & Ritter and Shahbaz, Charles & King on the X-ray light curve, we obtain a realistic mass of the disc at the peak of outburst to be Mh(0) , 1.5 × 1023 g (the distance d= 5 kpc) or 3.8 × 1022 g (d= 2.5 kpc). The disc radius at this moment is then Rh(0) , 2.5 × 1010 cm (d= 5 kpc) or 1.6 × 1010 cm (d= 2.5 kpc), provided that the factor f (the ratio of the mass of the hot disc at that moment with respect to its maximum possible mass) is close to unity. Even if we take a quite low f= 0.05, we still obtain Rh(0) by only 2.7 times larger. The reddening in the outburst maximum and brighter peak absolute magnitude of CI Cam with respect to those of soft X-ray transients in outbursts can be explained if the disc in CI Cam heats up an extended envelope and/or a strong jet is formed. We thus bring firm arguments for Robinson, Ivans & Welsh's hypothesis. On the other hand, we bring the arguments against the mass transfer burst from the donor and the periastron passage of the compact object. [source] Equilibria of a self-gravitating, rotating disc around a magnetized compact objectMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2004J. Ghanbari ABSTRACT We examine the effect of self-gravity in a rotating thick-disc equilibrium in the presence of a dipolar magnetic field. First, we find a self-similar solution for non-self-gravitating discs. The solution that we have found shows that the pressure and density equilibrium profiles are strongly modified by a self-consistent toroidal magnetic field. We introduce three dimensionless variables, CB, Cc and Ct, which indicate the relative importance of toroidal component of the magnetic field (CB), and centrifugal (Cc) and thermal (Ct) energy with respect to the gravitational potential energy of the central object. We study the effect of each of these on the structure of the disc. Secondly, we investigate the effect of self-gravity on the discs; thus, we introduce another dimensionless variable (Cg) which shows the importance of self-gravity. We find a self-similar solution for the equations of the system. Our solution shows that the structure of the disc is modified by the self-gravitation of the disc, the magnetic field of the central object and the azimuthal velocity of the gas disc. We find that self-gravity and magnetism from the central object can change the thickness and the shape of the disc. We show that as the effect of self-gravity increases the disc becomes thinner. We also show that, for different values of the star's magnetic field and of the disc's azimuthal velocity, the disc's shape and its density and pressure profiles are strongly modified. [source] Detection of an orbital period in the supergiant high-mass X-ray binary IGR J16465-4507 with Swift,BATMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010V. La Parola ABSTRACT We analysed the IGR J16465,4507 Burst Alert Telescope (BAT) survey data collected during the first 54 months of the Swift mission. The source is in a crowded field and it is revealed through an ad hoc imaging analysis at a significance level of ,14 standard deviations. The 15,50 keV average flux is . The timing analysis reveals an orbital period of 30.243 ± 0.035 d. The folded light curve shows the presence of a wide phase interval of minimum intensity, lasting ,20 per cent of the orbital period. This could be explained with a full eclipse of the compact object in an extremely eccentric orbit or with the passage of the compact source through a lower density wind at the orbit apastron. The modest dynamical range observed during the BAT monitoring suggests that IGR J16465,4507 is a wind-fed system, continuously accreting from a rather homogeneous wind, and not a member of the supergiant fast X-ray transient class. [source] The ultraluminous X-ray source in M82: an intermediate-mass black hole with a giant companionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006A. Patruno ABSTRACT The starburst galaxy M82, at a distance of 12 million light years, is the host of an unusually bright 2.4,16 × 1040 erg s,1 X-ray point source, which is best explained by an accreting black hole 102 to 104 times more massive than the Sun. Though the strongest candidate for a so-called intermediate-mass black hole, the only support stems from the observed luminosity and the 0.05,0.1 Hz quasi-periodicity in its signal. Interestingly, the 7,12 Myr old star cluster MGG-11 which has been associated with the X-ray source is sufficiently dense that an intermediate mass black hole could have been produced in the cluster core via collision runaway. The recently discovered 62.0 ± 2.5 d periodicity in the X-ray source X-1 further supports the hypothesis that this source is powered by a black hole several hundred times more massive than the Sun. We perform detailed binary evolution simulations with an accreting compact object of 10,5000 M, and find that the X-ray luminosity, the age of the cluster, the observed quasi-periodic oscillations and the now observed orbital period are explained best by a black hole of 200,5000 M, that accretes material from a 22,25 M, giant companion in a state of Roche-lobe contact. Interestingly, such a companion star is consistent with the expectation based on the tidal capture in a young and dense star cluster such as MGG-11, making the picture self-consistent. [source] Constraints on jet X-ray emission in low/hard-state X-ray binariesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2005Thomas J. Maccarone ABSTRACT We show that the similarities between the X-ray properties of low-luminosity accreting black holes and accreting neutron stars, combined with the differences in their radio properties, argue that the X-rays from these systems are unlikely to be formed in the relativistic jets. Specifically, the spectra of extreme island-state neutron stars and low/hard-state black holes are known to be quite similar, while the power spectra from these systems are known to show only minor differences beyond what would be expected from scaling the characteristic variability frequencies by the mass of the compact object. The spectral and temporal similarities thus imply a common emission mechanism that has only minor deviations from having all key parameters scaling linearly with the mass of the compact object, while we show that this is inconsistent with the observations that the radio powers of neutron stars are typically about 30 times lower than those of black holes at the same X-ray luminosity. We also show that an abrupt luminosity change would be expected when a system makes a spectral state transition from a radiatively inefficient jet-dominated accretion flow to a thin disc-dominated flow, but that such a change is not seen. [source] The X-ray source population of the Andromeda galaxy M 31,ASTRONOMISCHE NACHRICHTEN, Issue 2 2008W. Pietsch Abstract First studies of the X-ray source population of M 31 were performed with the Einstein Observatory and ROSAT. High resolution Chandra Observatory images not only spatially resolved the center area but also supernova remnants (SNRs) in the galaxy. Source catalogues of restricted areas were presented with high astrometric accuracy. Also luminosity function studies and studies of individual sources based on Chandra and XMM-Newton observations led to a better knowledge of the X-ray source population. An XMM-Newton source catalog based on archival observations revealed more than 850 sources down to a 0.2,4.5 keV luminosity of 1035 erg s,1. EPIC hardness ratios as well as informations from earlier X-ray, optical, and radio catalogues were used to distinguish between different source classes (SNRs, supersoft sources (SSSs), X-ray binaries (XRBs), globular cluster sources within M 31, and foreground stars and objects in the background). However, many sources could only be classified as "hard". These sources may either be XRBs or Crab-like SNRs in M 31 or background sources. Two of the globular cluster sources could be identified as low mass XRBs with a neutron star as compact object as they showed type I X-ray bursts. Many of the SSSs were identified as optical novae. Inspired by these results an XMM-Newton survey of the entire D25 disk of M 31 and a dedicated program to monitor X-ray counterparts of optical novae in M 31 was started. We discuss implications for further nearby galaxy studies. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Parallel operation of CartaBlanca on shared and distributed memory computersCONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 1 2004N. T. Padial-Collins Abstract We describe the parallel performance of the pure Java CartaBlanca code on heat transfer and multiphase fluid flow problems. CartaBlanca is designed for parallel computations on partitioned unstructured meshes. It uses Java's thread facility to manage computations on each of the mesh partitions. Inter-partition communications are handled by two compact objects for node-by-node communication along partition boundaries and for global reduction calculations across the entire mesh. For distributed calculations, the JavaParty package from the University of Karlsruhe is demonstrated to work with CartaBlanca. Copyright © 2004 John Wiley & Sons, Ltd. [source] Observation of interference effects in coherent diffraction of nanocrystals under X-ray standing-wave illuminationJOURNAL OF SYNCHROTRON RADIATION, Issue 6 2007Piotr Gryko Coherent X-ray diffraction is a useful technique for understanding the structure of compact objects including those which can be represented as phase objects. X-rays are highly penetrating and have wavelengths very close to atomic spacing. In this work, gold nanocrystals (on a reflecting substrate) were imaged at the Advanced Photon Source and found to produce a novel double diffraction pattern. Simulations were carried out to explain the experimental diffraction pattern in terms of reflection of the incident beam from the substrate to produce a standing wave. The experimental data were then phased to produce a two-dimensional real-space image of the gold. It is expected that the standing-wave illumination may be a useful tool to aid the convergence of the phasing algorithms for nanocrystals. [source] Equilibrium states of magnetized toroid,central compact object systemsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009Jun Otani ABSTRACT Equilibrium configurations of self-gravitating magnetized toroid,central compact object systems have been constructed in the framework of the Newtonian gravity. We have succeeded in including not only poloidal but also toroidal magnetic fields under the ideal magnetohydrodynamic approximation. We find two new and interesting results about the critical equilibrium states of such systems beyond which no equilibrium states are allowed to exist. First, there appear critical distances from the central compact objects to the inner surfaces of the magnetized toroids. Furthermore, these critical distances are much larger than the distances of the innermost stable circular orbits. It implies that even if these systems would be treated in the framework of general relativity, there would appear cusp structures of the effective total potential of the gravitational and magnetic forces for strongly magnetized toroids which are different from the general relativistic cusp structures. Secondly, since the strength of the magnetic field for the critical equilibrium configurations is roughly 1015 G if the mass of the central object is 1.4 M, and the maximum density of the toroid is 1011 g cm,3, the existence of equilibrium states of toroids around compact objects seems to set limit to the maximum value of the magnetic field of the system to be ,1015 G, i.e. no stronger magnetic fields can be realized for the systems consisting of magnetized toroids and central compact objects with the masses around the typical neutron star mass. The value of the maximum density of the toroid, 1011 g cm,3, is taken from the theoretical computational results of binary neutron star merging simulations in full general relativity. [source] Thermal evolution of a rotating strange star in the colour superconductivity phaseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Xiaoping Zheng ABSTRACT Under the combination effect of recommencement heating due to the spin-down of strange stars (SSs) and heat preservation due to the weak conduction heat of the crust, Cooper pair breaking and formation (PBF) in colour superconducting quark matter arises. We investigate the cooling of SSs with a crust in the colour superconductivity phase including both deconfinement heating (DH) and the PBF process. We find that DH can delay the thermal evolution of SSs and the PBF process suppresses the early temperature rise of the stars. The cooling SSs behave within the brightness constraint of young compact objects when the colour superconductivity gap is small enough. [source] Modelling the energy dependencies of X-ray quasi-periodic oscillations in accreting compact objectsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005Piotr T. ABSTRACT We have constructed models of the quasi-periodic variability of X-ray emission from accreting compact objects. Assuming a general scenario of a propagation model of variability, with inverse Compton upscattering as the emission mechanism, we have considered a number of cases for the periodic modulation: modulation of the plasma heating rate, cooling rate by external soft photons and the amplitude of the reprocessed component. We have computed various observational characteristics which can be compared to good quality data. These include Fourier-frequency resolved spectra and the results of cross-correlation analysis between light curves at different energies. Each model of modulation predicts specific observational signatures, which help in identifying the physical processes driving quasi-periodic oscillations emission in accreting sources. [source] X-ray beaming caused by resonance scattering in the accretion column of magnetic cataclysmic variablesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Y. Terada Extremely strong ionized Fe emission lines, with equivalent widths reaching ,4000 eV, were discovered by ASCA from a few Galactic compact objects, including AX J2315,0592, RX J1802.1+1804 and AX J1842.8,0423. These objects are thought to be binary systems containing magnetized white dwarfs (WDs). A possible interpretation of the strong Fe K line is the line-photon collimation in the WD accretion column, as a result of resonance scattering of line photons. The collimation occurs when the accretion column has a flat shape, and the effect is augmented by the vertical velocity gradient, which reduces the resonant trapping of resonant photons along the magnetic field lines. This effect was quantitatively confirmed with Monte Carlo simulations. Furthermore, with ASCA observations of the polar V834 Centauri, this collimation effect was clearly detected as a rotational modulation of the equivalent width of the Fe K emission line. The extremely strong emission lines mentioned above can be explained consistently by our interpretation. Combining this effect with other X-ray information, the geometry and plasma parameters in the accretion column were determined. [source] Comparisons and connections between mean field dynamo theory and accretion disc theoryASTRONOMISCHE NACHRICHTEN, Issue 1 2010E.G. Blackman Abstract The origin of large scale magnetic fields in astrophysical rotators, and the conversion of gravitational energy into radiation near stars and compact objects via accretion have been subjects of active research for a half century. Magnetohydrodynamic turbulence makes both problems highly nonlinear, so both subjects have benefitted from numerical simulations.However, understanding the key principles and practical modeling of observations warrants testable semi-analytic mean field theories that distill the essential physics. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory exemplify this pursuit. That the latter is a mean field theory is not always made explicit but the combination of turbulence and global symmetry imply such. The more commonly explicit presentation of assumptions in 20th century textbook MFDT has exposed it to arguably more widespread criticism than incurred by 20th century alpha-accretion theory despite complementary weaknesses. In the 21st century however, MFDT has experienced a breakthrough with a dynamical saturation theory that consistently agrees with simulations. Such has not yet occurred in accretion disc theory, though progress is emerging. Ironically however, for accretion engines, MFDT and accretion theory are presently two artificially uncoupled pieces of what should be a single coupled theory. Large scale fields and accretion flows are dynamically intertwined because large scale fields likely play a key role in angular momentum transport. I discuss and synthesize aspects of recent progress in MFDT and accretion disc theory to suggest why the two likely conspire in a unified theory (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||