arXiv subjects

P. T. Zycki

Publications and source records attributed to P. T. Zycki.

33 records · Page 2Linked to original sources

arXiv2002

Reprocessing of X-rays in AGN. I. Plane parallel geometry -- test of pressure equilibrium

We present a model of the vertical stratification and the spectra of an irradiated medium under the assumption of constant pressure. Such a solution has properties intermediate between constant density models and hydrostatic equilibrium models, and it may represent a flattened configuration of gas clumps accreting onto the central black hole. Such a medium develops a hot skin, thicker than hydrostatic models, but thinner than constant density models, under comparable irradiation. The range of theoretical values of the alpha_ox index is comparable to those from hydrostatic models and both are close to the observed values for Seyfert galaxies but lower than in quasars. The amount of X-ray Compton reflection is consistent with the observed range. The characteristic property of the model is a frequently multicomponent iron K alpha line.

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arXiv2001

The X-ray Spectrum of Cyg X-2

The spectra of disc accreting neutron stars generally show complex curvature, and individual components from the disc, boundary layer and neutron star surface cannot be uniquely identified. Here we show that much of the confusion over the spectral form derives from inadequate approximations for comptonization and for the iron line. There is an intrinsic low energy cutoff in comptonised spectra at the seed photon energy. It is very important to model this correctly in neutron star systems as these have expected seed photon temperatures (from either the neutron star surface, inner disc or self-absorbed cyclotron) of ~1 keV, clearly within the observed X-ray energy band. There is also reflected continuum emission which must accompany the observed iron line, which distorts the higher energy spectrum. We illustrate these points by a reanalysis of the GINGA spectra of Cyg X-2 at all points along its Z track, and show that the spectrum can be well fit by models in which the low energy spectrum is dominated by the disc, while the higher energy spectrum is dominated by comptonised emission from the boundary layer, together with its reflected spectrum from a relativistically smeared, ionised disc.

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arXiv2001

On the complex disc-corona interactions in the soft spectral states of Soft X-ray Transients

Accreting black holes show a complex and diverse behaviour in their soft spectral states. Although these spectra are dominated by a soft, thermal component which almost certainly arises from an accretion disc, there is also a hard X-ray tail indicating that some fraction of the accretion power is instead dissipated in hot, optically thin coronal material. During such states, best observed in the early outburst of Soft X-ray Transients, the ratio of power dissipated in the hot corona to that in the disc can vary from ~0 (pure disc accretion) to ~1 (equal power in each). Here we present results of spectral analyses of a number of sources, demonstrating the presence of complex features in their energy spectra. Our main findings are: (1) the soft components are not properly described by a thermal emission from accretion discs: they are appreciably broader than can be described by disc blackbody models even including relativistic effects, and (2) the spectral features near 5-9 keV commonly seen in such spectra can be well described by reprocessing of hard X-rays by optically thick, highly ionized, relativistically moving plasma.

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arXiv2001

Testing a model of variability of X-ray reprocessing features in Active Galactic Nuclei

A number of recent results from X-ray observations of Active Galactic Nuclei involving the Fe K alpha line (reduction of line variability compared to the X-ray continuum variability, the X-ray ``Baldwin effect'') were attributed to a presence of a hot, ionized skin of an accretion disc, suppressing emission of the line. The ionized skin appears as a result of the thermal instability of X-ray irradiated plasma. We test this hypothesis by computing the Thomson thickness of the hot skin on top of the 'alpha P_tot' Shakura-Sunyaev disc, by simultaneously solving the vertical structure of both the hot skin and the disc. We then compute a number of relations between observable quantities, e.g. the hard X-ray flux, amplitude of the observed reprocessed component, relativistic smearing of the K alpha line, the r.m.s. variability of the hard X-rays. These relations can be compared to present and future observations. We point out that this mechanism is unlikely to explain the behaviour of the X-ray source in MCG-6-30-15, where there is a number of arguments against the existence of a thick hot skin, but it can work for some other Seyfert 1 galaxies.

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arXiv2000

Probing the Inner Region of Cyg X-1 in the Low/Hard State through its X-ray Broad Band Spectrum

We present the broad band X-ray spectrum of Cyg X-1 in the low/hard state as observed by the instruments on board of BeppoSAX. The spectrum spans from 0.1 to 200 keV, allowing the total accretion luminosity to be observed, rather than extrapolated, corresponding to ~2 per cent of the Eddington limit for a 10 Solar mass black hole. The broad bandpass allows us to determine the continuum shape with great accuracy. Simple models of Compton up-scattering of seed photons from the accretion disk do not adequately match the spectrum. At low energies an additional continuum component is required, giving a complex soft excess which extends up to ~4 keV, in line with previous results from ASCA. Moreover we clearly detect a reflected component from the accretion disk which is smeared, probably because of relativistic and Doppler effects. The reflecting material is not strongly ionized and does not subtend a large solid angle as seen from the corona. The inner radius of the disk, that depends on the inclination of the system, is most probably between 10 and 70 gravitational radii (Rg). An unsmeared reprocessed component, probably originating from the companion star or the outer disk, could also be present. In this case, the inner radius of the disk, as inferred from the smeared reflection, is smaller, between 6 and 20 Rg.

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arXiv2000

X-ray reprocessing in Narrow-Line Seyfert 1 Galaxies: Ton S180 and Ark 564

We present the results of spectral analysis of the ASCA data for the Narrow-Line Seyfert 1 galaxy (NLS1) Ton S180 and simultaneous ASCA and RXTE data modelling for the NLS1 Ark 564. We model both the primary and reflected continuum as well as the iron K alpha line, the energy of which depends on the ionization state of the reprocessor. We show that the reprocessing matter is mildly ionized, and we find the soft to hard luminosity ratio to be about 2.5. The accretion rate approximately corresponds to the Eddington limit value.

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arXiv2000

Disk/corona model: The transition to ADAF

We propose a model of the accretion flow onto a black hole consisting of the accretion disk with an accreting two-temperature corona. The model is based on assumptions about the radiative and conductive energy exchange between the two phases and the pressure equilibrium. The complete model is determined by the mass, the accretion rate, and the viscosity parameter. We present the radial dependencies of parameters of such a two-phase flow, with advection in the corona and the disk/corona mass exchange due to evaporation/condensation included, and we determine the transition radius from a two-phase disk/corona accretion to a single-phase optically thin flow (ADAF) in the innermost part of the disk as a function of accretion rate. We identify the NLS1 galaxies with objects accreting at a rate close to the Eddington accretion rate. The strong variability of these objects may be related to the limit cycle behaviour expected in this luminosity range, as the disk, unstable due to the dominance by the radiation pressure, oscillates between the two stable branches: the advection-dominated optically thick branch and the evaporation branch.

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arXiv2000

Structure of the Circumnuclear Region of Seyfert 2 Galaxies Revealed by RXTE Hard X-ray Observations of NGC 4945

We report the Rossi X-ray Timing Explorer observations of the heavily absorbed Seyfert 2 galaxy NGC 4945, which confirm the large column density of the absorber, corresponding to the optical depth to Thomson scattering of about 2.4. The hard X-ray flux (8-30 keV) measured by us is rapidly variable with a large amplitude, on time scales of a day or less. This implies that the bulk of the extreme absorption does not originate in a parsec-size, geometrically thick molecular torus: limits on the amount of scattered flux require that the optically thick material on parsec scales must be rather geometrically thin, subtending a half-angle of less than 10 degrees as seen from the nucleus. This presents a problem in accounting for the Cosmic X-ray Background primarily with AGN possessing such geometry.

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arXiv2000

The relativistic iron line profile in the Seyfert 1 Galaxy IC4329a

We present simultaneous ASCA and RXTE data on the bright Seyfert 1 galaxy IC4329a. The iron line is significantly broadened, but not to the extent expected from an accretion disk which extends down to the last stable orbit around a black hole. We marginally detect a narrow line component, presumably from the molecular torus, but even including this gives a line profile from the accretion disk which is significantly narrower that that seen in MCG-6-30-15, and is much more like that seen from the low/hard state galactic black hole candidates. This is consistent with the inner disk being truncated before the last stable orbit, forming a hot flow at small radii as in the ADAF models. However, we cannot rule out the presence of an inner disk which does not contribute to the reflected spectrum, either because of extreme ionisation suppressing the characteristic atomic features of the reflected spectrum or because the X-ray source is intrinsically anisotropic, so it does not illuminate the inner disk. The source was monitored by RXTE every 2 days for 2 months, and these snapshot spectra show that there is intrinsic spectral variability. The data are good enough to disentangle the power law from the reflected continuum and we see that the power law softens as the source brightens. The lack of a corresponding increase in the observed reflected spectrum implies that either the changes in disk inner radial extent/ionisation structure are small, or that the variability is actually driven by changes in the seed photons which are decoupled from the hard X-ray mechanism.

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arXiv1998

On accretion flow in Soft X-ray Transients

We have analyzed archival Ginga data of a number of Soft X-ray Transient sources and modelled them in an attempt to constrain the geometry and physical properties of accretion. Our models include a self-consistent description of X-ray reprocessing, linking the properties of the reflected continuum and those of the iron fluorescence Kαline, and allowing for relativistic smearing of spectral features. We derive constraints on the inner extent of the disk and its ionization, assuming a generic geometry of a central X-ray source and an external accretion disk. Fitting our models to the data we have found that in some sources - GS 1124-68 (Nova Muscae 1991) and GS 2000+25 - the evolution during decline proceeded in qualitative agreement with recent theoretical models based on advection-dominated solutions of accretion flow. These models link the high/soft-low/hard transition with a change of geometry, most importantly, the inner radius of the optically thick disk. Quantitatively, our analysis requires serious revision of the model assumptions. A remarkably exceptional case is that of GS 2023+338, where the source behavior both during outburst and decline was quite different than the other, more ``standard'' ones. However, we found that the source did go into the high/soft state, contrary to claims in literature.

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arXiv1997

Relativistic smearing of the reflection spectrum in Galactic Black Hole Candidates

We identify the reflected component in the GINGA spectra of Nova Muscae, a Black Hole transient system which has been used as the prototype for the recent advection dominated disk models. We see that the reflected spectrum is generally significantly relativistically smeared, and use this and the amount of reflection to track the innermost extent of the accretion disk. We see that the optically thick disk does retreat during the decline, but more slowly than predicted by the advective models, posing problems for this description of the accretion flow.

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arXiv1997

Geometry of accretion in Soft X-ray Transients

We present results of modelling Ginga data of a number of Soft X-ray Transient sources performed with the aim of constraining the geometry of accretion during various stages of the sources' evolution. Assuming a generic geometry of a central, extended X-ray source with an external, optically thick accretion disk, we use consistent models of X-ray reprocessing to constrain the inner radius of the accretion disk and its ionization state. We show that the evolution of GS 1124-68 (Nova Muscae 1991) and GS 2000+25 agree qualitatively with the recent ideas linking the high/low state transition with changing the inner radius of the optically thick disk, but the case of GS 2023+33 (V404 Cyg) poses a problem for any model.

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arXiv1996

Luminosity to the Eddington luminosity ratio in AGN

We discuss the optical/UV/Xray spectra of AGN within the frame of the corona model of Witt, Czerny and Zycki (1996). In this model both the disk and the corona accrete and release energy through the viscosity. The relative strength the the disk and the corona is therefore determined by the model. Translated into the spectra, this model allows to predict the spectral index alpha_{ox}, measuring the relative strenth of the big blue bump with respect to the hard X-ray power law. Comparison of the predicted and observed distributions of alpha_{ox} allow us to conclude that the luminosity to the Eddington luminosity ratio in quasars cover typically the range 0.01 - 0.1 and it is broader in Seyferts, 0.001 - 0.3, or even higher. We identify Narrow Line Seyfert galaxies as objects having high luminosity to the Eddington luminosity ratio although, in principle, the second branch of high big blue bump objects for this ratio being below 0.001 is also predicted.

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arXiv1995

Accretion Discs with Accreting Coronae in AGN - II. Nuclear Wind

We study an accretion disc with a hot continuous corona. We assume that the corona itself accretes and therefore it is powered directly by the release of the gravitational energy and cooled by radiative interaction with the disc. We consider the vertical structure of such a corona and show that the radial infall is accompanied by strong vertical outflow. Such a model has two important consequences: (i) at a given radius the corona forms only for accretion rate larger than a limiting value and the fraction of energy dissipated in the corona decreases with increasing accretion rate, and (ii) the disc spectra are significantly softer in the optical/UV band in comparison with the predictions of standard accretion discs due to the mass loss and the decrease of internal dissipation in the disc. Both trends correspond well to the mean spectra of radio quiet AGN and observed luminosity states in galactic black hole candidates.

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arXiv1995

ACCRETION DISCS WITH ACCRETING CORONAE IN AGN. I. SOLUTIONS IN HYDROSTATIC EQUILIBRIUM

We discuss a model for the central region of radio-quiet AGN in which coronal accretion is the source of energy for X-ray emission. We start from the Haardt & Maraschi model but supplement it with pressure balance condition, two possibilities of the angular momentum transfer and three models of the disc-corona transition. We employ the alpha-description of viscosity within the two layers. Both descriptions of the angular momentum transport lead to similar conclusions about the fraction of energy liberated in the disc although they predict strongly different properties of the disc interiors. The coupled disc/corona system has one family of solutions at low accretion rates, two families of solutions at intermediate accretion rates and no solution for high accretion rate. Of the two families of solutions, the first one has weakly dissipating coronae and the second one has coronae which dominate energetically the system. The two solutions merge at sub-Eddington accretion rate because a corona in hydrostatic equilibrium can dissipate only a limited amount of energy, which depends on the viscosity parameter.

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