Papers by Demosthenes Kazanas
The Astrophysical Journal, 2013
We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation ... more We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, θ), ionization parameter ξ (r, θ), and velocity structure v(r, θ) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvénic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfvén surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, ξ , and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.
The Astrophysical Journal, 2006
We have obtained Fourier-resolved spectra of the black-hole binary 4U 1543-47 in the canonical st... more We have obtained Fourier-resolved spectra of the black-hole binary 4U 1543-47 in the canonical states (high/soft, very high, intermediate and low/hard) observed in this source during the decay of an outburst that took place in 2002. Our objective is to investigate the variability of the spectral components generally used to describe the energy spectra of black-hole systems, namely a disk component, a power-law component attributed to Comptonization by a hot corona and the contribution of the iron line due to reprocessing of the high energy (E > ∼ 7 keV) radiation. We find that i) the disk component is not variable on time scales shorter than ∼ 100 seconds, ii) the reprocessing emission as manifest by the variability of the Fe Kα line responds to the primary radiation variations down to time scales of ∼ 70 ms in the high and very-high states, but longer than 2 s in the low state, iii) the low-frequency QPOs are associated with variations of the X-ray power law spectral component and not to the disk component and iv) the spectra corresponding to the highest Fourier frequency are the hardest (show the flatter spectra) at a given spectral state. These results question models that explain the observed power spectra as due to modulations of the accretion rate alone, as such models do not provide any apparent reason for a Fourier frequency dependence of the power law spectral indices.
The multifrequency spectral evolution of blazar 3C 345
The Astrophysical Journal, 1994
... The 2 mm data also show a high flux level after the 1991 optical outburst, but no short times... more ... The 2 mm data also show a high flux level after the 1991 optical outburst, but no short timescalevariations are seen in the flux curve. ... 2.-Long-term UV and optical flux curves are plotted including all available archival IUE data of 3C 345 and the RHO B-band observations. ...
The Astrophysical Journal, 2001
We test the "reprocessing paradigm" of the optical-UV AGN variability, according to which the var... more We test the "reprocessing paradigm" of the optical-UV AGN variability, according to which the variations in this wavelength range are driven by a variable X-ray component, by detailed modeling of the correlated X-ray-optical (λλ 3590 and 5510Å) variability of the recent multiwavelength campaign of NGC 3516. To this end we produce model optical light curves by convolving the observed X-ray flux with the response function of an infinite, thin accretion disk, illuminated by a point-like X-ray source at a given height h X above the compact object (the lamppost model) and compare their properties (amplitude, morphology, lags) to those observed. Special attention is given to the correct computation of the X-ray albedo of the disk, by including an X-ray heated ionized layer in hydrostatic equilibrium on its surface. We further compute the X-ray reflection response at two energies (E = 1, 20 keV) and argue for the possibility of hard lags in their cross spectra. We also compute the continuum Optical-UV and the X-ray reflection spectra as well as the Fe Kα fluorescent line profiles which we also compare to observations. Despite the large (≃ 50%) amplitude excursions of the X-ray flux, the model optical light curves exhibit variability amplitudes of 3-4 %, not unlike those observed. The model light curves exhibit clearly a feature associated with a large X-ray flux excursion not seen in the data, arguing for a reprocessing region of size > ∼ 10 15 cm, even though no lags are discernible in the cross correlation functions (CCF) of the λ 3590 and λ 5510Å model light curves. However, the (CCF) between the X-ray and the model optical variations show clear lags of 0.1 and 0.25 days for black hole masses M = 10 7 , 10 8 M ⊙ respectively, not apparent in the data. The synchrony of X-ray-optical variations points toward the smaller mass values (M = 10 7 M ⊙), which however are inconsistent with the model X-ray reflection spectra which exhibit several features below ∼ 4 keV and above ≃ 9 keV not observed in the data. In fact, the model X-ray reflection spectra are roughly consistent with observations only for M > ∼ 10 8 M ⊙. The observed Fe Kα line profiles are too broad to be consistent with our models. Lowering the distance of the X-ray source from the disk, h X , helps a little in this respect, but only at the expense of reducing unrealistically the amplitude of Optical-UV variations. Our conclusion is that the combination of the observed optical/UV/X-ray spectral and timing observations are inconsistent with the lamppost model geometry for NGC 3516.
The Astrophysical Journal, 2002
We perform a first study of time-dependent X-ray reflection in photo-ionized accretion disks. We ... more We perform a first study of time-dependent X-ray reflection in photo-ionized 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 relaxation time scale of the problem since it affects evolution of the ionization state of the reflector. Because of this the Fe Kα line emissivity depends on the shape and intensity of the illuminating flux in prior times, and hence it is not a function of the instantaneous illuminating spectrum. Moreover, during the transition, a prominent Helium-like component of the Fe Kα line may appear. As a result, the Fe Kα line flux may appear to be completely uncorrelated with X-ray continuum flux on time scales shorter than the dynamical time. In addition, the time-dependence of the illuminating flux may leave imprints even on the time-averaged Fe Kα line spectra, which may be used as an additional test of accretion disk geometry. Our findings appear to be important for the proposed Fe Kα line reverberation studies in lamppost-like geometries for accretion rates exceeding about ∼ 1% of the Eddington value. However, most AGN do not show Heliumlike lines that are prominent in such models, probably indicating that these models are not applicable to real sources.
The Astrophysical Journal, 2000
We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretio... 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.
![Research paper thumbnail of Narrow Moving F[CLC]e[/CLC] Kα Lines from Magnetic Flares in Active Galactic Nuclei](https://attachments.academia-assets.com/98418508/thumbnails/1.jpg)
The Astrophysical Journal, 2001
We point out that luminous magnetic flares, thought to occur in standard AGN accretion disks, can... more We point out that luminous magnetic flares, thought to occur in standard AGN accretion disks, cannot be located much higher than few pressure scale heights above the disk. Using this fact, we estimate the fraction of the disk surface illuminated by a typical flare. This fraction turns out to be very small for geometrically thin disks, which implies that the instantaneous Fe Kα emission line from a specific magnetic flare is narrow. The line is red-or blue-shifted depending on the position of the observer relative to the flare and sweeps across the line band with time. We present several examples of theoretical time-resolved line profiles from such flares for Schwarzschild geometry. The observations of such moving features with future X-ray telescopes will present a powerful test of the accretion disk geometry and may also test General Relativity in the strong field limit.
The Astrophysical Journal, 2015
We present a study of X-ray ionization of magnetohydrodynamic (MHD) accretion-disk winds in an ef... more We present a study of X-ray ionization of magnetohydrodynamic (MHD) accretion-disk winds in an effort to constrain the physics underlying the highlyionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various sub-classes of Seyfert active galactic nuclei (AGNs). Our primary focus is to show that magnetically-driven outflows are indeed physically plausible candidates for the observed outflows accounting for the AGN absorption properties of the present X-ray spectroscopic observations. Employing a stratified MHD wind launched across the entire AGN accretion disk, we calculate its X-ray ionization and the ensuing X-ray absorption line spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds to model the absorption features in an XMM-Newton/EPIC spectrum of the narrow-line Seyfert, PG 1211+143. We find, through identifying the detected features with Fe Kα transitions, that the absorber has a characteristic ionization parameter of log(ξ c [erg cm s −1 ]) ≃ 5 − 6 and a column density on the order of N H ≃ 10 23 cm −2 , outflowing at a characteristic velocity of v c /c ≃ 0.1 − 0.2 (where c is the speed of light). The best-fit model favors its radial location at r c ≃ 200R o (R o is the black hole innermost stable circular orbit), with an inner wind truncation radius at R t ≃ 30R o. The overall K-shell feature in the data is suggested to be dominated by Fe xxv with very little contribution from Fe xxvi and weakly-ionized iron, which is in a good agreement with a series of earlier analysis of the UFOs in various AGNs including PG 1211+143.
The Astrophysical Journal, 2001
Advection-dominated accretion flows (ADAFs) have a positive Bernoulli parameter, and are therefor... more Advection-dominated accretion flows (ADAFs) have a positive Bernoulli parameter, and are therefore gravitationally unbound. The Newtonian ADAF model has been generalized recently to obtain the ADIOS model that includes outflows of energy and angular momentum, thereby allowing accretion to proceed self-consistently. However, the utilization of a Newtonian gravitational potential limits the ability of this model to describe the inner region of the disk, where any relativistic outflows are likely to originate. In this paper we modify the ADIOS scenario to incorporate a pseudo-Newtonian potential, which approximates the effects of general relativity. The analysis yields a unique, self-similar solution for the structure of the coupled disk/wind system. Interesting features of the new solution include the relativistic character of the outflow in the vicinity of the radius of marginal stability, which represents the inner edge of the
Relativistic Outflows from ADAFs
ABSTRACT
We present the 2D ionization structure of self-similar MHD winds off accretion disks around irrad... more We present the 2D ionization structure of self-similar MHD winds off accretion disks around irradiated by a central X-ray source. Based on earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r)~1/r. We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. Particular attention is paid to the absorption measure distribution (AMD), namely their Hydrogen-equivalent column per logarithmic ionization parameter \xi interval, d N_H/(d \log \xi), which provides a measure of the winds' radial density profiles. For n(r)~1/r the AMD is found to be independent of \xi, in good agreement with its behavior inferred from the X-ray spectra of several AGNs. For the specific wind structure and X-ray spectrum we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v~100-300 km/sec (at \log \xi~2-3) for Fe XVII and v~1,000-4,000 km/sec (at \log \xi~4-5) for Fe XXV, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate \dot{m} and LOS angle \theta. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing \dot{m} and increases steeply with \theta. As such, we concur with previous authors that these wind configurations, viewed globally, incorporate all the requisite properties of the parsec scale "torii" invoked in AGN unification schemes. We indicate that a combination of the AMD and absorption line profile observations can uniquely determine these model parameters and their bearing on AGN population demographics.
Astrophysical Journal, 2002
We perform the first study of time-dependent X-ray reflection in photoionized accretion disks. We... 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
The Astrophysical Journal, 2010
We extend our modeling of the ionization structure of magnetohydrodynamic (MHD) accretion-disk wi... more We extend our modeling of the ionization structure of magnetohydrodynamic (MHD) accretion-disk winds, previously applied to Seyfert galaxies, to a population of quasi-stellar-objects (QSOs) of much lower X-ray-to-UV flux ratios, i.e. smaller α ox index, motivated by UV/X-ray ionized absorbers with extremely high outflow velocities in UV-luminous QSOs. We demonstrate that magneticallydriven winds ionized by a spectrum with α ox ≃ −2 can produce the charge states responsible for C iv and Fe xxv/Fe xxvi absorption in wind regions with corresponding maximum velocities of v(C iv) < ∼ 0.1c and v(Fe xxv) < ∼ 0.6c (where c is the speed of light) and column densities N H ∼ 10 23 − 10 24 cm −2 , in general agreement with observations. In contrast to the conventional radiation-driven wind models, high-velocity flows are always present in our MHD-driven winds but manifest in the absorption spectra only for α ox < ∼ −2, as larger α ox values ionize the wind completely out to radii too large to demonstrate the presence of these high velocities. We thus predict increasing velocities of these ionized absorbers with decreasing (steeper) α ox , a quantity that emerges as the defining parameter in the kinematics of the AGN UV/X-ray absorbers.
Astrophysical Journal - ASTROPHYS J, 2010
We present the two-dimensional ionization structure of self-similar magnetohydrodynamic winds off... more We present the two-dimensional ionization structure of self-similar magnetohydrodynamic winds off accretion disks around and irradiated by a central X-ray point source. On the basis of earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r) vprop 1/r (r is the spherical radial coordinate). We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. We focus our attention on the distribution of the column density of the various ions as a function of the ionization parameter xi (or equivalently r) and the angle theta. Particular attention is paid to the absorption measure distribution (AMD), namely their hydrogen-equivalent column per logarithmic xi interval, dN H/dlog xi, which provides a measure of the winds' radial density profiles. For the chosen density pr...
We present a unified model for the structure and appearance of accretion powered sources across t... more We present a unified model for the structure and appearance of accretion powered sources across their entire luminosity range from galactic X-ray binaries to luminous quasars, with emphasis on AGN and their phenomenology. Central to this model is the notion of MHD winds launched from the accretion disks that power these objects. These winds provide the matter that manifests as blueshifted absorption features in the UV and X-ray spectra of a large fraction of these sources; furthermore, their density distribution in the poloidal plane determines the "appearance" (i.e. the column and velocity structure of these absorption features) as a function of the observer inclination angle. This work focuses on just the broadest characteristics of these objects; nonetheless, it provides scaling laws that allow one to reproduce within this model the properties of objects spanning a very wide luminosity range and viewed at different inclination angles, and trace them to a common underlyi...
The Astrophysical Journal, 2007
We describe the methodology and compute the illumination of geometrically thin accretion disks ar... more We describe the methodology and compute the illumination of geometrically thin accretion disks around black holes of arbitrary spin parameter a exposed to the radiation of a point-like, isotropic source at arbitrary height above the disk on its symmetry axis. We then provide analytic fitting formulae for the illumination as a function of the source height h and the black hole angular momentum a. We find that for a source on the disk symmetry axis and h/M > 3, the main effect of the parameter a is allowing the disk to extend to smaller radii (approaching r/M → 1 as a/M → 1) and thus allow the illumination of regions of much higher rotational velocity and redshift. We also compute the illumination profiles for anisotropic emission associated with the motion of the source relative to the accretion disk and present the fractions of photons absorbed by the black hole, intercepted by the disk or escaping to infinity for both isotropic and anisotropic emission for a/M = 0 and a/M = 0.99. As the anisotropy (of a source approaching the disk) increases the illumination profile reduces (approximately) to a single power-law, whose index, q, because of absorption of the beamed photons by the black hole, saturates to a value no higher than q 3. Finally, we compute the fluorescence Fe line profiles associated with the specific illumination and compare them among various cases.
Astrophysical Journal, 1999
We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretio... 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...
Astrophysical Journal, 1999
We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretio... 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...
Hubble Space Telescope ultraviolet spectroscopy of blazars: emission-line properties and black hole masses
Monthly Notices of the Royal Astronomical Society, 2005
The Astrophysical Journal, 1998
The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underw... more The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ∼10 in amplitude) flare in γ-rays towards the end of a 3-week pointing by CGRO, in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ∼8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSAT and IUE and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of ≃3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ∼1 day. The optical-UV light curves, which are not well sampled during the high energy flare, exhibit more modest variations (factor of ∼2) and a lower degree of correlation. The flux at millimetric wavelengths was near an historical maximum during the γ-ray flare peak and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical Stanford, CA 94305
Uploads
Papers by Demosthenes Kazanas