demosthenes kazanas
NASA Goddard, Gravitational Astrophysics Laboraatory, Faculty Member
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It is generally considered that the X-ray emission in AGN and Galactic Black Hole Candidates is produced by flares above the surface of a geometrically thin optically thick accretion disk, which extends down to the Innermost Stable... more
It is generally considered that the X-ray emission in AGN and Galactic Black Hole Candidates is produced by flares above the surface of a geometrically thin optically thick accretion disk, which extends down to the Innermost Stable Circular Orbit (ISCO) of the black hole. We consider the influence of the black hole geometry on the light curves of these flares. To this end we follow a large number of photon orbits emitted impulsively in a locally isotropic fashion, at any phase of the disk orbit and examine their arrival times at infinity by an observer near the plane of the disk. We find out that the presence of the black hole spin induces a certain delay in the photon arrivals, as prograde photon orbits reach the observer on shorter (on the average) times than the retrograde ones. We form a histogram of the differences in photon time arrivals and we find that it exhibits several well defined peaks depending on the flare position and the black hole spin separated by $\Delta t \simeq 30 M$, where M is the black hole mass. The peaks disappear as the spin parameter goes to zero, implying that one could in principle measure the value of the black hole spin with timing measurements of sufficiently high signal to noise ratio.
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We propose a scheme that accounts for the broader spectral and temporal properties of galactic black hole X-ray transients. The fundamental notion behind this proposal is that the mass accretion rate, \(\dot{M}\), of the disks of these... more
We propose a scheme that accounts for the broader spectral and temporal properties of galactic black hole X-ray transients. The fundamental notion behind this proposal is that the mass accretion rate, \(\dot{M}\), of the disks of these systems depends on the radius, as it has been proposed for ADIOS. We propose that, because of this dependence of \(\dot{M}\) on radius, an accretion disk which is geometrically thin and cool at large radii converts into a geometrically thick, advection dominated, hot disk interior to a transition radius at which the local accretion rate drops below the square of the viscosity parameter, a condition for the existence of advection dominated flows. We argue also that such a transition requires in addition that the vertical disk support be provided by magnetic fields. As discussed in other chapters of this book, the origin of these fields is local to the disk by the Poynting Robertson battery, thereby providing a complete self-contained picture for the spectra and evolution of these systems.
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We model the correlated X-ray -- UV observations of NGC 7469, for which well sampled data in both these bands have been obtained recently in a multiwavelength monitoring campaign. To this end we derive the transfer function in wavelength... more
We model the correlated X-ray -- UV observations of NGC 7469, for which well sampled data in both these bands have been obtained recently in a multiwavelength monitoring campaign. To this end we derive the transfer function in wavelength l and time lag t, for reprocessing hard (X-ray) photons from a point source to softer ones (UV-optical) by an infinite plane (representing a cool, thin accretion disk) located at a given distance below the X-ray source, under the assumption that the X-ray flux is absorbed and emitted locally by the disk as a black body of temperature appropriate to the incident flux. Using the observed X-ray light curve as input we have computed the expected continuum UV emission as a function of time at several wavelengths ( 1315 A, 6962 A, 15000 A, 30000 A) assuming that the X-ray source is located one schwarzchild radius above the disk plane, with the mass of the black hole M and the latitude angle θ of the observer relative to the disk plane as free parameters. ...
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We model the correlated X-ray - UV observations of NGC 7469, for which well sampled data in both these bands have been obtained recently by a multiwavelength monitoring campaign. To this end we derive the response function for... more
We model the correlated X-ray - UV observations of NGC 7469, for which well sampled data in both these bands have been obtained recently by a multiwavelength monitoring campaign. To this end we derive the response function for reprocessing hard (X-ray) to UV radiation from a point source by an infinite plane, located at a given distance below the source, as a function of the wavelength lambda of the reprocessed radiation and the time lag tau , under the assumption that the X-ray flux is reprocessed and emitted locally as a black body of the appropriate temperature. Using the observed X-ray light curve as input we compute the expected continuum UV emission as a function of time at wavelengths lambda lambda 1360 Angstroms and lambda lambda 4690 Angstroms, assuming that the the X-ray source is located a few Schwarzschild radii above the disk plane, with the mass of the black hole as a free parameter. We searched the parameter space of black hole masses and observer azimuthal angles but...
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Page 1. Steadily rotating perfect-fluid gravitating prolate spheroids in Weyl gravity This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1997 Class. Quantum Grav. 14 2663... more
Page 1. Steadily rotating perfect-fluid gravitating prolate spheroids in Weyl gravity This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1997 Class. Quantum Grav. 14 2663 (http://iopscience.iop.org/0264-9381/14/9/020) ...
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We present, for the first time, the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to... more
We present, for the first time, the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, ${\bf E\cdot B}=0$, everywhere. The unique distribution of electric current along the open magnetic field lines which is required for the solution to be continuous and smooth is obtained numerically. With the geometry of the field lines thus determined we compute the dynamics of the associated MHD wind. The main result is that the relativistic outflow contained in the magnetosphere is not accelerated to the extremely relativistic energies required for the flow to generate gamma rays. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely when either the force-free and/or the ideal MHD condition ${\bf E\cdot B}=0$ are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.
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Using the exact solution of the axisymmetric pulsar magnetosphere derived in a previous publication and the conservation laws of the associated MHD flow, we show that the Lorentz factor of the outflowing plasma increases linearly with... more
Using the exact solution of the axisymmetric pulsar magnetosphere derived in a previous publication and the conservation laws of the associated MHD flow, we show that the Lorentz factor of the outflowing plasma increases linearly with distance from the light cylinder. Therefore, the ratio of the Poynting to particle energy flux, generically referred to as sigma, decreases inversely proportional to
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We examine the effects of a relativistic wind on the spin down of a neutron star and apply our results to the study of Soft Gamma Repeaters (SGRs), thought to be neutron stars with magnetic fields > 10^{14} G. We derive a spin-down... more
We examine the effects of a relativistic wind on the spin down of a neutron star and apply our results to the study of Soft Gamma Repeaters (SGRs), thought to be neutron stars with magnetic fields > 10^{14} G. We derive a spin-down formula that includes torques from both dipole radiation and episodic or continuous particle winds. We find that if SGR1806-20 puts out a continuous particle wind of 10^{37} {\rm erg} {\rm s}^{-1}, then the pulsar age is consistent with that of the surrounding supernova remnant, but the derived surface dipole magnetic field is only 3 \times 10^{13} G, in the range of normal radio pulsars. If instead, the particle wind flows are episodic with small duty cycle, then the observed period derivatives imply magnetar-strength fields, while still allowing characteristic ages within a factor of two of the estimated supernova remnant age. Close monitoring of the periods of SGRs will be able to establish or place limits on the wind duty cycle and thus the magnetic field and age of the neutron star.
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With the use of the equations of motion of massless fields moving in a curved Friedmann-Robertson-Walker universe, we show, in some simple cases, that the energy-momentum tensor of a maximally 3-space symmetric distribution of the fields... more
With the use of the equations of motion of massless fields moving in a curved Friedmann-Robertson-Walker universe, we show, in some simple cases, that the energy-momentum tensor of a maximally 3-space symmetric distribution of the fields (i.e., an incoherent averaging over a complete set of modes of the field propagating in a Robertson-Walker background) has the standard perfect fluid form. As far as we know such an explicit demonstration, as well as the establishment of the compatibility of the equations of motion of the gravitational field with such an incoherently averaged source in the standard cosmology, has not previously been presented in the literature. Our results are found to hold for any value of the spatial curvature of the universe.
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A general method for determining the structure of the gravitational equations of motion is presented in the fourth-order theory of gravity based on local conformal Weyl invariance of the gravitational action. The explicit structure for... more
A general method for determining the structure of the gravitational equations of motion is presented in the fourth-order theory of gravity based on local conformal Weyl invariance of the gravitational action. The explicit structure for these equations is given for a time-dependent, spherically symmetric geometry.
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We study the interior structure of a locally conformal invariant fourth order theory of gravity in the presence of a static, spherically symmetric gravitational source. We find, quite remarkably, that the associated dynamics is determined... more
We study the interior structure of a locally conformal invariant fourth order theory of gravity in the presence of a static, spherically symmetric gravitational source. We find, quite remarkably, that the associated dynamics is determined exactly and without any approximation at all by a simple fourth order Poisson equation which thus describes both the strong and weak field limits of the theory in this static case. We present the solutions to this fourth order equation and find that we are able to recover all of the standard Newton-Euler gravitational phenomenology in the weak gravity limit, to thus establish the observational viability of the weak field limit of the fourth order theory. Additionally, we make a critical analysis of the second order Poisson equation, and find that the currently available experimental evidence for its validity is not as clearcut and definitive as is commonly believed, with there not apparently being any conclusive observational support for it at all either on the very largest distance scales far outside of fundamental sources, or on the very smallest ones within their interiors. Our study enables us to deduce that even though the familiar second order Poisson gravitational equation may be sufficient to yield Newton's Law of Gravity it is not in fact necessary.
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... In the Weyl theory, as already noted by Mannheim (1988) the de Sitter solution is a vacuum (7 ... y measures all departures of the Weyl theory from that of Einstein with a cosmological term, and ... parameters /3 and k, the parameter... more
... In the Weyl theory, as already noted by Mannheim (1988) the de Sitter solution is a vacuum (7 ... y measures all departures of the Weyl theory from that of Einstein with a cosmological term, and ... parameters /3 and k, the parameter y should be related to the interior dynamics of the ...
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We perform the first study of time-dependent X-ray reflection in photoionized accretion disks. We assume a step-functional change in the X-ray flux and use a simplified prescription to describe the time evolution of the illuminated gas... more
We perform the first study of time-dependent X-ray reflection in photoionized accretion disks. We assume a step-functional change in the X-ray flux and use a simplified prescription to describe the time evolution of the illuminated gas density profile in response to changes in the flux. We find that the dynamical time for readjustment of the hydrostatic balance is an important
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We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretion disks valid at any radius and for a broad range of spectral indices of the incident X-rays, accretion rates and black hole masses. We... more
We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretion disks valid at any radius and for a broad range of spectral indices of the incident X-rays, accretion rates and black hole masses. We expect that this formula may find useful applications in studies of geometry of the inner part of accretion flows around compact objects, and in several other astrophysically important problems, such as the recently observed X-ray ``Baldwin'' effect (i.e., monotonic decrease of Fe line's equivalent width with the X-ray luminosity of AGN), the problem of missing Lyman edge in AGN, and line and continuum variability studies in accretion disks around compact objects. We compute the reflected X-ray spectra for several representative cases and show that for hard X-ray spectra and large ionizing fluxes the skin represents a perfect mirror that does not produce any Fe lines or absorption features. At the same time, for soft X-ray spectra or small ionizing fluxes, the skin produces very strong ionized absorption edge and highly ionized Fe lines that should be observable in the reflected spectra.
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We study the X-ray illumination of an accretion disk. We relax the simplifying assumption of constant gas density used in most previous studies; instead we determine the density from hydrostatic balance. It is found that the thermal... more
We study the X-ray illumination of an accretion disk. We relax the simplifying assumption of constant gas density used in most previous studies; instead we determine the density from hydrostatic balance. It is found that the thermal ionization instability prevents the illuminated gas from attaining temperatures at which the gas is unstable. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature ($\sim 10^7 - 10^8$ K); at larger depths, the gas temperature drops abruptly to form a thin layer with $T\sim 10^6$ K, while at yet larger depths it decreases sharply to the disk effective temperature. We find that most of the Fe K$\alpha$ line emission and absorption edge are produced in the coolest, deepest layers, while the Fe atoms in the hottest, uppermost layers are generally almost fully ionized, hence making a negligible contribution to reprocessing features in $\sim 6.4-10$ keV energy range. We provide a summary of how X-ray reprocessing features depend on parameters of the problem. The results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. Therefore, we conclude that X-ray reflection calculations should always utilize hydrostatic balance in order to provide a reliable theoretical interpretation of observed X-ray spectra of AGN and GBHCs.
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We have found relations between the temporal and spectral properties of radiation Comptonized in an extended atmosphere associated with compact accreting sources. We demonstrate that the fluctuation power spectrum density (PSD) imposes... more
We have found relations between the temporal and spectral properties of radiation Comptonized in an extended atmosphere associated with compact accreting sources. We demonstrate that the fluctuation power spectrum density (PSD) imposes constraints on the atmosphere scale and profile. Furthermore, we indicate that the slope and low-frequency break of the PSD are related to the Thomson depth, τ0, of the atmosphere and the radius of its physical size, respectively. Since the energy spectrum of the escaping radiation also depends on τ0 (and the electron temperature kTe), the relation between spectral and temporal properties follows. This relation allows, for the first time, an estimate of the accreting matter Thomson depth, τ0, independent of arguments involving Comptonization. We present figures for the light curves and PSDs of different energy bands, the photon energy spectra, and the phase lags as functions of the variability frequency. The temporal properties of the high (soft) and low (hard) state of black hole sources are discussed in this context.